Relationship among types of nerve growth factor receptors on PC12 cells

We analyzed the kinetics and thermodynamics of 125I-nerve growth factor (125I-NGF) binding to NGF-receptor on PC12 cells. We used conditions of pseudo-first order kinetics and techniques to quantitate internalized complexes, "slow" or high affinity binding complexes, and cell surface "fast" or low affinity complexes. Two possible models were examined: binding to two independent receptors at the cell surface (i.e. high and low affinity forms of NGF-receptor) and a model for consecutive formation of fast, low affinity binding followed by slow, high affinity binding or internalization. Our data are consistent with the consecutive model only. The rates of association and dissociation of NGF with slow, high affinity sites and internalized, acid wash-resistant sites are indistinguishable from each other. We also analyzed, in detail, the two assays primarily used to distinguish slow binding complexes from internalized complexes. Scatchard analysis of total binding and dissociation of pre-equilibrated 125I-NGF in the presence of unlabeled NGF at high concentration (cold wash). Neither of these assays shows any evidence that the slow, high affinity binding step is different from internalization of the 125I-NGF-receptor complex. Based on this analysis, there are only two detectable forms of NGF-receptor on PC12 cells: complexes on the surface of the cells with a binding affinity of 0.5 nM at 37 degrees C and complexes internalized by the cells. Furthermore, the data are consistent with a model in which NGF-receptor is internalized constitutively and independently of occupancy by NGF. We also examined the fate of internalized 125I-NGF. In the first 60 min after contact with PC12 cells, no degradation of 125I-NGF was observed. Moreover, a significant amount of 125I-NGF recirculates to the cell surface and is released as intact, Mr = 13,000 NGF. The cells were also stimulated by NGF in a primary neurite outgrowth assay with an ED50 of 2-16 pM under conditions of low initial cell numbers in a large extracellular volume of NGF-containing medium. Thus, low level occupancy of the cell surface receptors, Kd = 0.5 nM, for several days is sufficient to stimulate neurite outgrowth. This indicates the presence of spare NGF-receptors on the surface PC12 cells.     

A comparison of binding properties and structure of NGF receptor on PC12 pheochromocytoma and A875 melanoma cells

Rat PC12 pheochromocytoma and human A875 melanoma cells express nerve growth factor (NGF) receptors on their surfaces. Covalent crosslinking of bound 125I-NGF to PC12 or A875 intact cells or plasma membrane-enriched fractions resulted in labelling of a peptide doublet at Mr = 110,000 and a single labelled peptide at Mr = 200,000 for each of the cell and membrane preparations. However, a difference between equilibrium binding properties of NGF-receptor on PC12 and A875 cells was observed. PC12 cells exhibited biphasic binding properties with two apparent binding sites: KD = 5.2 nM sites and KD = 0.3 nM sites. The high-affinity PC12 binding sites were trypsin resistant, and 125I-NGF dissociated slowly from them. A875 cells exhibited sites with homogeneous properties (KD = 1.0 nM), all binding sites were trypsin sensitive, and 125I-NGF dissociated rapidly in the presence of unlabelled NGF. Membrane-enriched fractions from either cell type contained binding sites with a uniform low affinity (KD = 3 nM) that were trypsin sensitive, and 125I-NGF rapidly dissociated from them. Sixty to 80 percent of binding sites in membranes could be converted to the high-affinity, trypsin-resistant state by addition of wheat germ agglutinin (WGA). The loss of high-affinity, trypsin-resistant sites from PC12 cells during preparation of plasma membrane fractions does not appear to be the result of selective isolation of low-affinity sites or proteolytic degradation since there is a loss of 125I-NGF binding immediately after cell lysis which is not blocked by protease inhibitors. Also, high-affinity, trypsin-resistant binding sites are not found associated with other cell fractions. The differences between receptor properties on PC12 cells and on A875 cells apparently are the result of differences in the respective intracellular environments. Thus, significant structural homology exists between receptors on A875 and PC12 cells. Cell components other than the binding unit of the NGF receptor may be responsible for the different properties of receptor.     

Purification of the receptor for nerve growth factor from A875 melanoma cells by affinity chromatography

The receptor for nerve growth factor (NGF) has been purified to near homogeneity from octylglucoside extracts of A875 melanoma cell membranes by the use of repetitive affinity chromatography on NGF-Sepharose. Elution of purified receptor (NGF receptor) was accomplished with 0.15 M NaCl, pH 11.0, containing phosphatidylcholine and octylglucoside. Chromatography on two columns of NGF-Sepharose yielded a 1500-fold purification of the receptor, as assessed by 125I-NGF binding, and permitted recovery of 9% of the total binding activity in the soluble extract. Scatchard analysis of equilibrium binding of 125I-NGF provided similar Kd values for NGF receptors in soluble extracts of A875 membranes (2.2 nM) and with purified NGF receptor (3.1 nM). Examination of NGF receptor after electrophoresis on sodium dodecyl sulfate-polyacrylamide gels revealed the presence of two major peptides, of Mr = 85,000 and Mr = 200,000. Affinity labeling experiments, done with 125I-NGF and A875 cells, soluble extracts of A875 cell membranes, and purified receptor, show that both of these components of the NGF receptor can be specifically cross-linked to 125I-NGF.     

Molecular investigations on the high-affinity nerve growth factor receptor.

Nerve growth factor (NGF) receptors have been investigated by means of affinity labeling with 125I-NGF and chemical cross-linking. Two distinct NGF-receptor complexes are detected on PC12 cells; these correspond to 100 kd and 158 kd for the low-affinity (LNGFR) and the high-affinity (HNGFR) receptors, respectively. Interestingly, three different antibodies directed against distinct epitopes on the LNGFR immunoprecipitate the low-but not the high-affinity NGF-receptor complex. Although the identities of the signaling molecules in the HNGFR are unknown, antibodies to the src, ras, raf-1, and yes products fail to immunoprecipitate either receptor complex, suggesting that these molecules are not a part of, or tightly coupled to, either receptor type. Phosphotyrosine residues are found exclusively on the HNGFR complex, suggesting that tyrosine phosphorylation may be one of the initiating events in the NGF-induced signal transduction cascade.     

Nerve growth factor receptor from rabbit sympathetic ganglia membranes. Relationship between subforms

The receptor for nerve growth factor (NGF) was purified from Triton X-100 extracts of sympathetic ganglia membranes by affinity chromatography on NGF-Sepharose. Elution of purified receptor was accomplished at pH 5 in the presence of 1 M NaCl. Sodium dodecyl sulfate gel electrophoresis of the purified iodinated receptor showed three major bands at Mr = 126,000, Mr = 105,000, and Mr = 81,000. Affinity labeling of the purified receptor using 125I-NGF and the photoreactive agent N-hydroxysuccinimidyl-p-azidobenzoate resulted in two major cross-linked complexes corresponding to Mr = 135,000 and Mr = 110,000. This labeling pattern is similar to that observed with sympathetic ganglia membranes (Massague, J., Guillette, B. J., Czech, M. P., Morgan, C. J., and Bradshaw, R. A. (1981) J. Biol. Chem. 256, 9419-9424) and indicates that these two forms do not arise from the cross-linking procedure. Reaction of the photoaffinity labeled NGF receptors with increasing amounts of trypsin resulted in a progressive decrease in the high molecular weight complex with a concomitant increase in the low molecular weight form. When the larger complex was isolated by electroelution from a sodium dodecyl sulfate gel and treated with trypsin, a species corresponding to Mr = 100,000 was generated. These observations are best explained by a precursor-product relationship for the two NGF receptor species of sympathetic neurons.     

Characterization of nerve growth factor binding to cultured neural crest cells: evidence of an early developmental form of the NGF receptor

Cultured neural crest cells undergoing differentiation have been shown to contain a subpopulation of cells with specific receptors for nerve growth factor (NGF). These cells are the potential targets of NGF during differentiation and development. This study was done to pharmacologically characterize the binding of NGF to long-term (1- to 3-week) cultures of quail neural crest cells. The data indicate that 125I-NGF binding was specific and saturable, with less than 20% nonspecific binding. Scatchard analysis revealed the presence of one type (class) of receptors with a binding constant (Kd) similar to that of the low-affinity binding site described for embryonic dorsal root and sympathetic ganglia (approximately 3.2 nM). This was corroborated by displacement experiments (Kd of 1.3 nM), in which 125I-NGF binding was measured in the presence of increasing concentrations of nonradioactive NGF. In addition, affinity labeling revealed that the 125I-NGF-receptor complex had a molecular weight of about 93K, characteristic of the low-affinity NGF receptor of PC12 cells. The NGF receptor of cultured neural crest cells was trypsin-sensitive, as is typical of the low-affinity NGF binding sites. These findings indicate that differentiating neural crest cells lack high-affinity 125I-NGF binding sites. In contrast, embryonic dorsal root and sympathetic ganglia cells, known NGF targets, have both high- and low-affinity receptors. Measurements of the differential release of surface-bound 125I-NGF indicated that a relatively small amount (about 14%) of NGF is internalized over a 1-hr period. Cultured neural crest cells which bear NGF receptors were also shown by light microscopic radioautographic techniques to incorporate [3H]thymidine. I suggest, therefore, that cultured neural crest cells which have not terminally differentiated, as judged by morphological criteria and continued proliferation, may express an early developmental form of the NGF receptor.     

Biochemical evidence that the type II insulin-like growth factor receptor is identical to the cation-independent mannose 6-phosphate receptor

Cloning and sequencing of the human type II insulin-like growth factor (IGF) receptor cDNA revealed an 80% deduced amino acid sequence homology with the bovine cation-independent mannose 6-phosphate (Man-6-P) receptor, suggesting identity of the two receptors (Morgan, D. O., Edman, J. C., Standring, D. N., Fried, V. A., Smith, M. C., Roth, R. A., and Rutter, W. J. (1987) Nature 329, 301-307). We have performed biochemical experiments that support this proposal. Rat liver type II IGF receptor, purified by the conventional method of IGF-II affinity chromatography, bound quantitatively to a beta-galactosidase affinity column and was eluted with Man-6-P. Bovine liver Man-6-P receptor, prepared by the conventional method of affinity chromatography on phosphomannan-Sepharose, bound IGF-II with high affinity (Kd = 1 nM). Affinity cross-linking of 125I-IGF-II to the Man-6-P receptor and analysis by sodium dodecyl sulfate-gel electrophoresis showed that beta-galactosidase, but not Man-6-P, inhibited the formation of the 250-kDa 125I-IGF-II-receptor complex. The inhibition by beta-galactosidase was prevented by coincubation with Man-6-P. 125I-IGF-II did not bind to the 46-kDa cation-dependent Man-6-P receptor. For immunologic studies we purified type II IGF receptors and Man-6-P receptors in parallel from rat placental membranes using either IGF-II- or beta-galactosidase affinity chromatography. A panel of five antisera that previously had been raised against either type II IGF receptor or Man-6-P receptor behaved identically toward type II IGF receptor versus Man-6-P receptor in ligand blocking and immunoprecipitation assays. Our data support the conclusion that the type II IGF receptor and the cation-independent Man-6-P receptor are the same protein and that the IGF-II and Man-6-P-binding sites are distinct.     

Insulin-like growth factor II binding to the type I somatomedin receptor. Evidence for two high affinity binding sites

We have previously shown that the antireceptor antibody alpha IR-3 inhibits binding of 125I-somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) to the 130-kDa alpha subunit of the type I receptor in human placental membranes, but does not block 125I-insulin-like growth factor II (IGF-II) binding to a similar 130-kDa complex in these membranes. To determine whether the 130-kDa 125I-IGF-II binding complex represents a homologous receptor or whether 125I-IGF-II binds to the type I receptor at a site that is not blocked by alpha IR-3, type I receptors were purified by affinity chromatography on Sepharose linked alpha IR-3. The purified receptors bound both 125I-Sm-C/IGF-I and 125I-IGF-II avidly (KD = 2.0 X 10(-10) M and 3.0 X 10(-10) M, respectively). The maximal inhibition of 125I-Sm-C/IGF-I binding by the antibody, however, was 62% while only 15% of 125I-IGF-II binding was inhibited by alpha IR-3. In the presence of 500 nM alpha IR-3, Sm-C/IGF-I bound with lower affinity (KD = 6.5 X 10(-10) M) than IGF-II (KD = 4.5 X 10(-10) M) and IGF-II was the more potent inhibitor of 125I-Sm-C/IGF-I binding. These findings suggest that the type I receptor contains two different binding sites. The site designated IA has highest affinity for Sm-C/IGF-I and is blocked by alpha IR-3. Site IB has higher affinity for IGF-II than for Sm-C/IGF-I and is not blocked by alpha IR-3.     

A single Mr approximately 103,000 125I-beta-nerve growth factor-affinity-labeled species represents both the low and high affinity forms of the nerve growth factor receptor

Both high and low affinity receptors for nerve growth factor (NGF) have been described, but only the former appear to mediate NGF actions and uptake. To specifically characterize the molecular identity of the high affinity site and to compare it with the low affinity site, the water-soluble carbodiimide EDC was used to cross-link 125I-NGF to NGF receptors on: rat PC12 cells, PC12nnr5 cells (PC12 mutants that have only low affinity NGF binding), SH-SY5Y human neuroblastoma cells (which have only high affinity binding sites), and cultured rat sympathetic ganglion cells. A variety of criteria were used to distinguish the two classes of affinity-labeled receptors: competition with unlabeled NGF, dissociation rate, and selective solubilization by 0.1% Triton X-100. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that cross-linking generated only a single Mr approximately 103,000 125I-NGF affinity-labeled species which represents both the low and high affinity forms of the receptor. The 125I-NGF X receptor complexes formed with both affinity classes of the receptor were quantitatively immunoprecipitated by the monoclonal anti-NGF-receptor antibody 192-IgG and both showed identical shifts in mobility when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. These findings indicate that both high and low affinity NGF receptors possess apparently identical NGF-binding moieties. The differences between the kinetic and functional properties of the two receptor types may therefore result from their interactions with other membrane components or with cytoplasmic proteins.     

Identification of heparan sulfate proteoglycan as a high affinity receptor for acidic fibroblast growth factor (aFGF) in a parathyroid cell line.

We have characterized two high affinity acidic fibroblast growth factor (aFGF) receptors in a rat parathyroid cell line (PT-r). Affinity labeling with 125I-aFGF showed that these two receptors, apparent molecular masses, 150 and 130 kDa, respectively, display higher affinity for aFGF than for bFGF. The 150-kDa receptor bears a heparan sulfate chain(s), demonstrated by a decrease in size of 15-20 kDa with heparitinase digestion after affinity labeling. Heparitinase digestion before affinity labeling markedly reduced the intensity of the 150 kDa species. Scatchard analysis showed two different high affinity binding sites (Kd of 3.9 pM with 180 sites/cell and Kd of 110 pM with 5800 sites/cell). The higher affinity site was completely eliminated by digestion with heparitinase before adding labeled aFGF; the lower affinity site was unaffected. In ion exchange chromatography after metabolic labeling of the cells with [3H]glucosamine and affinity labeling with 125I-aFGF, the larger receptor-ligand complex, 165 kDa, eluted with approximately 0.5 M NaCl, typical eluting conditions for heparan sulfate proteoglycans. Both of the receptor-ligand complexes were smaller on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than two major heparan sulfate proteoglycans, HSPG I and II, which we characterized in this cell line previously (Yanagishita, M., Brandi, M. L., and Sakaguchi, K. (1989) J. Biol. Chem. 264, 15714-15720). Both receptors have similar N-linked oligosaccharide and sialic acid contents, shown by analysis of affinity-labeled receptors upon digestion with glycopeptidase F and with neuraminidase. All together, these results suggest that PT-r cells bear two distinct high affinity receptors for aFGF, a 150-kDa receptor which is a heparan sulfate proteoglycan and another that is a glycoprotein. The heparan sulfate glycosaminoglycan moiety of the 150- kDa receptor is critical for high affinity binding of aFGF. These findings contrast with current concepts derived from other systems, suggesting that heparan sulfate glycosaminoglycans/proteoglycans function as a reservoir source for FGF or as a group of low affinity binding sites.     

Molecular characteristics of nerve growth factor receptors on PC12 cells

Cross-linking of 125I-nerve growth factor (NGF) to PC12 cells with the photoreactive heterobifunctional agent N-hydroxysuccinimidyl-4-azidobenzoate results in the labeling of two major bands with Mr 158,000 and 100,000 and a minor band with Mr 225,000 as determined by polyacrylamide gel electrophoresis under denaturing and reducing conditions. Binding of 125I-NGF to and cross-linking into all these species is abolished in the presence of excess unlabeled NGF but not in the presence of unlabeled epidermal growth factor, insulin, or bovine pancreatic trypsin inhibitor. When PC12 cells with bound 125I-NGF are incubated in excess unlabeled NGF at 0 degree C prior to cross-linking, only the Mr 158,000 species remains. In addition, binding of 125I-NGF to the Mr 158,000 complex is trypsin-resistant, whereas binding to the Mr 100,000 complex is not. These experiments identify the Mr 158,000 species as the slow NGF-receptor complex (chase stable at 0 degree C) and the smaller Mr 100,000 species as the fast NGF-receptor complex (trypsin sensitive). Furthermore, 125I-NGF bound to the former but not to the latter species is displaced by very-low concentrations of NGF, showing that at least a significant fraction of the high-molecular-weight slow receptor is also a high-affinity receptor. This identification is supported by the finding that chick sensory neurons which possess both high- and low-affinity receptors exhibit two major labeled bands with Mr 145,000 and 105,000 as a result of cross-linking with 125I-NGF, whereas a cell population enriched in non-neuronal cells, which possess only low-affinity receptors, exhibits only the Mr 105,000 component. A shift in molecular weight of both species after pretreatment with neuraminidase indicates that both complexes contain sialoglycoproteins and rules out the possibility that differences in sialic acid content are responsible for the difference in molecular weight of the two complexes. The relative amount of the labeling of these two complexes is not affected by the presence of protease inhibitors nor by a variation of 5000-fold in cross-linker concentration. These results place some limits on possible models for the NGF receptors and their interconversion.     

Oocytes from the mutant restricted ovulator hen lack receptor for very low density lipoprotein

Hens of the "Restricted Ovulator" (R/O) chicken strain are characterized by the absence of egg-laying and concomitant severe hyperlipidemia due to a single gene defect (Ho, K. J., Lawrence, W. D., Lewis, L. A., Liu, L. B., and Taylor, C. B. (1974) Arch. Pathol. 98, 161-172). However, the underlying biochemical defect has not been identified. Previous studies on receptor-mediated growth of chicken oocytes have led to the characterization of a 95-kDa oocyte plasma membrane receptor that binds very low density lipoproteins (VLDL) (George, R., Barber, D. L., and Schneider, W. J. (1987) J. Biol. Chem. 262, 16838-16847). The current experiments demonstrate the absence of this receptor from R/O oocytes. Ligand binding experiments showed that ovarian membranes from mutant hens failed to display high affinity, saturable, and specific binding of 125I-VLDL. Ligand blotting with 125I-VLDL and Western blotting with polyclonal anti-receptor antibodies visualized the 95-kDa receptor in normal oocytes, but R/O ovarian membranes were devoid of any cross-reactive protein. Finally, plasma clearance of intravenously injected 125I-VLDL was dramatically impaired in R/O in comparison to normal hens, with a concomitant decrease in the radioactivity accumulating in R/O oocytes. These data strongly suggest that the absence of the 95-kDa receptor for VLDL from oocytes is responsible for the R/O phenotype, and that the receptor not only binds VLDL, but also mediates its uptake. This animal model provides a powerful tool for investigations of receptor-mediated growth of chicken oocytes and for the elucidation of regulatory mechanisms in lipid and lipoprotein metabolism of laying hens.     

Purification of the rat hepatic alpha 2-macroglobulin receptor as an approximately 440-kDa single chain protein

alpha 2-Macroglobulin-trypsin complex (alpha 2M.T) and alpha 2M-methylamine bind in a Ca2+-dependent way to a 400- to 500-kDa receptor in rat and human liver membranes (Gliemann, J., Davidsen, O., and Moestrup, S. K. (1989) Biochim. Biophys. Acta 980, 326-332). Here we report the preparation of alpha 2M receptors from rat liver membranes solubilized in 3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonic acid (CHAPS) dihydrate and incubated with Sepharose-immobilized alpha 2M-methylamine. The receptor preparation eluted with EDTA (pH 6.0) contained a protein larger than the 360-kDa alpha 2M (nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and some minor contaminants. The reduced large protein was about 440 kDa using reduced laminin (heavy chain: 400 kDa) as a standard. About 10 micrograms of receptor protein was obtained from 100 mg of liver membranes. The receptor preparation immobilized on nitrocellulose sheets bound 125I-alpha 2M.T, and the binding activity co-eluted with the 440-kDa protein. 125I-Labeled rat alpha 1-inhibitor-3 (alpha 1I3), a 200-kDa analogue of the alpha 2M subunit which binds to the alpha 2M receptors, was cross-linked to the 440-kDa protein. The receptor preparation was iodinated, and the 125I-labeled 440-kDa protein was isolated. It showed Ca2+-dependent saturable binding to alpha 2M-methylamine. In conclusion, we have purified the major hepatic alpha 2M receptor as an approximately 440-kDa single chain protein.     

Hormone-induced conformational changes in the hepatic insulin receptor

The insulin receptor can exist in either a lower or a higher affinity state. Hormone binding alters the equilibrium between the two states of the insulin receptor, favoring the formation of that of higher affinity (Corin, R.E., and Donner, D.B. (1982), J. Biol. Chem. 257, 104-110). After brief or extended incubations with hormone, during which the fraction of higher affinity receptors increased, 125I-insulin was covalently coupled to the alpha subunits of its receptor using disuccinimidyl suberate. Some 125I-insulin remained bound to higher affinity receptors after dissociation of hormone from lower affinity sites. This hormone could also be covalently coupled to the alpha subunit of the receptor. During extended incubations between 125I-insulin and liver plasma membranes, components of the receptor were cleaved to yield degradation products of 120,000 and 23,000 Da. The significance of this process remains undetermined. Unoccupied insulin receptors were cleaved by trypsin to produce fragments of 94,000 and 37,000 Da which remained membrane-bound and could be covalently coupled to 125I-insulin. Trypsin treatment after binding yielded an additional receptor fragment of 64,000 Da. As the incubation time between 125I-insulin and membranes was lengthened, components of the receptor became progressively less sensitive to trypsin. Higher affinity binding sites isolated after release of rapid dissociating insulin were less sensitive to trypsin than were mixtures of higher and lower affinity receptors. These observations suggest that hormone binding produces two conformational changes (alterations of tryptic lability) in the hepatic insulin receptor. The first change is rapid and exposes parts of the receptor to tryptic degradation. The second, slower conformational change renders the receptor less sensitive to trypsin and occurs with the same time course as the increase of receptor affinity mediated by site occupancy.     

Characterization of the solubilized charybdotoxin receptor from bovine aortic smooth muscle.

Monoiodotyrosine ([125I]ChTX) binds with high affinity to a single class of receptors present in bovine aortic smooth muscle sarcolemmal membranes that are functionally associated with the high-conductance Ca(2+)-activated K+ channel [maxi-K channel; Vázquez, J., et al. (1989) J. Biol. Chem. 265, 20902-20909]. Cross-linking experiments carried out with this preparation in the presence of [125I]ChTX and disuccinimidyl suberate indicate specific incorporation of radioactivity into a protein of Mr 35,000. The smooth muscle ChTX receptor can be solubilized in active form in the presence of selected detergents. Treatment of membranes with digitonin releases about 50% of the ChTX binding sites. The solubilized receptor retains the same biochemical and pharmacological properties that are characteristic of toxin interaction with membrane-bound receptors. The solubilized receptor binds specifically to wheat germ agglutinin-Sepharose resin, suggesting that it is a glycoprotein. Functional ChTX binding sites can also be solubilized in 3-[(3-cholamidopropyl)dimethylamino]-1-propanesulfonate (CHAPS). Sucrose density gradient centrifugation of either digitonin or CHAPS extracts indicates that the ChTX receptor has a high apparent sedimentation coefficient (s20,w = 23 and 18 S, respectively). Cross-linking experiments indicate that the appearance of the 35-kDa membrane protein correlates with ChTX binding activity after both wheat germ agglutinin-Sepharose and sucrose density gradient centrifugation steps. Given the high apparent sedimentation coefficient of the ChTX receptor, the 35-kDa membrane protein may be a subunit of a higher molecular weight complex which forms the maxi-K channel in smooth muscle sarcolemma.     

Insulin-like growth factor I receptors in retinal rod outer segments

We have previously reported that the GDP-bound alpha-subunit of the GTP-binding protein transducin, present in outer segments of retinal rod cells (ROS), serves as a high affinity in vitro substrate (Km = 1 microM) for the insulin receptor kinase. The present study demonstrates that transducin also serves as in vitro substrate for an endogenous IGF-I receptor kinase isolated from ROS membranes. The presence of insulin-like growth factor I (IGF-I) receptors in ROS is evident from the high affinity and specific binding of 125I-IGF-I to ROS membranes (Kd = 3 nM) which contain 110 fmol of IGF-I binding sites/mg of membrane protein. Furthermore, cross-linking of 125I-IGF-I labels the 135-kDa alpha-subunit of this receptor. 125I-Insulin binding capacity to ROS membranes is less than 5% that of IGF-I. The IGF-I-stimulated tyrosine kinase activity in solubilized and partially purified receptors from ROS autophosphorylates its own 95-kDa beta-subunits as well as other substrates like transducin. Insulin, which is 200-fold less potent than IGF-I in competing for 125I-IGF-I binding, is only 5-fold less potent than IGF-I in stimulating the receptor kinase activity. This suggests that insulin is much more potent than IGF-I in coupling ligand binding with kinase activation. The previously reported presence of IGF-I in the vitreous, together with our present studies, strongly suggest that the IGF-I receptor kinase, through phosphorylation of endogenous proteins like transducin, could play a role in mediating transmembrane signal transduction in ROS.     

Production of a monoclonal antibody directed against the nerve growth factor receptor from sympathetic membranes

Spleen cells from BALB/c mice immunized with a plasma membrane-enriched fraction from rabbit sympathetic ganglia were fused with the mouse myeloma NS1. A hybrid clone was obtained that produced monoclonal antibody directed against the receptor for nerve growth factor (NGF). The antibody, identified as IgG, was able to immunoprecipitate solubilized NGF receptor in the presence or absence of bound NGF. The antibody bound specifically to sympathetic membranes with high affinity but did not affect the binding of 125I-NGF to its receptor in sympathetic or sensory neurons or PC12 cells.     

Characterization of a membrane regulator of insulin receptor affinity

Using the technique of radiation inactivation we have previously shown that the insulin receptor behaves as if it is composed of at least two functional components: a binding component (Mr approximately equal to 100,000) and an affinity regulatory component (Mr approximately equal to 300,000). The interaction between the affinity regulator and binding component results in a decrease in the affinity of the receptor for insulin. To examine in more detail the interaction between this "affinity regulator" and the binding component we have studied the insulin receptor by radiation inactivation under conditions which alter receptor concentration or receptor affinity. Liver membranes of ob/ob mice exhibit a decrease in insulin binding when compared to their lean litter mates which is due to a decrease in receptor concentration. When studied by radiation inactivation, however, there was no detectable change in the interaction or size of the two receptor components. By contrast, under circumstances in which the affinity of the receptor was increased (treatment with high salt, high pH, 1 mM dithiothreitol, 1-5 micrograms/ml of trypsin), the interaction between the regulatory and binding components was either decreased or absent, i.e. there was no increase in binding with irradiation. Conversely, conditions which produce a decrease in receptor affinity resulted in an increase in the interaction between the regulatory and binding components. The changes in receptor affinity and interactions of the two components produced by either high salt or pH were reversible. Partial purification of the solubilized receptor on lectin affinity columns resulted in the apparent removal of the affinity regulator, i.e. receptor affinity was increased. In this state, radiation inactivation studies revealed a monoexponential decay indicating no interaction between binding and regulatory components. Taken together, these results suggest that the affinity regulator is a membrane protein which is both trypsin-sensitive and has disulfide bond(s) essential for its function. The interaction between the affinity regulator and binding component is not via a covalent bond and the two components appear to be separated by lectin chromatography. The interaction between these components appears to be altered in most states associated with altered receptor affinity.     

Isolation of the bombesin/gastrin-releasing peptide receptor from human small cell lung carcinoma NCI-H345 cells.

Purification of the gastrin-releasing peptide (GRP) or bombesin receptor has proved elusive in part due to technical difficulties. In the present studies, the problem of oxidized radioligand was avoided by the use of 125I-GRP, which was verified to be not oxidized by high performance liquid chromatography. Specific 125I-GRP binding (at 0 degrees C) to intact human small cell lung carcinoma NCI-H345 cells which had been subjected to a dilute acid wash was 6 fmol/10(6) cells. Inhibition of GRP degradation by human H345 cell membranes through the use of phenanthroline or phosphoramidon permitted the development of binding assays for the GRP receptor in detergent-solubilized crude membrane preparations. The solubilized GRP receptor exhibited saturable, high affinity (KD = 1.3 nM), temperature-dependent specific binding averaging 402 +/- 65 fmol/mg protein (mean +/- S.E. for eight separate membrane preparations with 125I-GRP concentration = 3 nM), with a Bmax = 434 fmol/mg protein using a gel filtration binding assay. That the GRP receptor had been solubilized was demonstrated by its failure to pellet when centrifuged at 100,000 x g for 60 min, its passage through a 0.22-micron filter without loss of binding activity, and its elution in the void volume of a Sephadex G-50 gel filtration column, but within the inclusion volume of a Sephacryl S-200 column (Ve/V0 = 1.1). Isolation of the GRP receptor from human H345 cell-solubilized membranes was achieved by ligand affinity chromatography. A unique 70-kDa band on silver-stained reduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis was reproducibly eluted from GRP14-27 affinity columns by an acidic high salt buffer, but binding activity was denatured by these conditions. The protein nature of the GRP receptor was demonstrated by its sensitivity to proteases after isolation. In addition, two unique bands of 65 and 70 kDa were eluted from the GRP14-27 affinity column with GRP14-27 in neutral buffer, and this eluate possessed specific 125I-GRP binding with a stoichiometry of approximately 1:1. Thus, reported here is the isolation of a functional membrane-associated, saturable, high affinity GRP receptor with temperature-dependent binding from the solubilized membranes of human H345 cells.     

The glycocalicin portion of platelet glycoprotein Ib expresses both high and moderate affinity receptor sites for thrombin. A soluble radioreceptor assay for the interaction of thrombin with platelets

A soluble radioreceptor assay has been developed to characterize thrombin receptor activities of the human platelet membrane. 125I-Thrombin was added to platelet membranes solubilized in 1% Triton X-100, and thrombin bound to platelet receptors was separated from free thrombin by precipitation with wheat germ agglutinin (WGA) in the presence of alpha 1-acid glycoprotein as carrier. Both high affinity binding (Ki, 0.09 nM; R1, 0.30 pmol/mg protein) and moderate affinity binding (K2, 38 nM; R2, 72 pmol/mg protein) were detected in the detergent-solubilized membrane preparations and these binding parameters were in excellent agreement with values previously determined using intact platelets (Harmon, J. T., and Jamieson, G. A. (1985) Biochemistry 24, 58-64). Using the soluble radioreceptor assay, both high and moderate affinity binding was detected in highly purified preparations of glycoprotein Ib (GPIb) and glycocalicin, and the binding isotherms were identical with those of the crude detergent-solubilized membrane preparation. Treatment of detergent-solubilized membranes with increasing concentrations of a monospecific polyclonal antibody to glycocalicin resulted in the stepwise depletion of GPIb and concomitant reductions of thrombin binding activity. These results demonstrate that both high and moderate affinity binding of thrombin to platelets is completely expressed in the glycocalicin portion of GPIb.