Multiple forms of human dopamine beta-hydroxylase in SH-SY5Y neuroblastoma cells

Dopamine beta-hydroxylase exists as three forms in human neuroblastoma (SH-SY5Y) cells. The membrane-bound form of the hydroxylase contains three different species with apparent relative molecular weights of 73,000, 77,000, and 82,000. The intracellular soluble form of dopamine beta-hydroxylase was present as a single species with an apparent molecular weight of 73,000. Pulse-chase experiments showed that membranous dopamine beta-hydroxylase contains two subunit forms of 73,000 and 77,000 after short chase times. The soluble hydroxylase was synthesized as a single species of 73,000 at approximately the same rate as the lower molecular weight species of the membranous enzyme. A constitutively secreted third form of the enzyme with an intermediate apparent molecular weight also incorporated [35S]sulfate, whereas no significant amount of [35S]sulfate was observed in the cellular forms of the enzyme. The [35S]sulfate was incorporated on N-linked oligosaccharides. Approximately 12% of the enzyme is released constitutively within 1 h. These results demonstrate that neuronal cells have the ability to constitutively secrete a specific form of dopamine beta-hydroxylase which may contribute to the levels of this enzyme found in plasma.     

Sulfation and constitutive secretion of dopamine beta-hydroxylase from rat pheochromocytoma (PC12) cells

The biosynthesis and secretion of dopamine beta-hydroxylase were investigated by radiolabeling rat pheochromocytoma (PC12) cells in culture. Intracellular dopamine beta-hydroxylase from a crude chromaffin vesicle fraction and secreted dopamine beta-hydroxylase from culture medium were immunoprecipitated using antiserum made against purified bovine soluble dopamine beta-hydroxylase. Analysis of the immunoprecipitated enzyme on sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that: 1) the membrane-bound form of the hydroxylase from crude secretory vesicle membrane extracts contained two nonidentical subunits in approximately stoichiometric amounts (Mr = 77,000 and 73,000); 2) the soluble hydroxylase from the lysate of these secretory vesicles was composed predominantly of a single subunit (Mr = 73,000); and 3) the hydroxylase secreted into the medium under resting conditions was also composed of a single subunit (approximate Mr = 73,000). All subunits of the multiple forms of hydroxylase were glycoproteins. Under resting conditions, the rate of secretion of hydroxylase was approximately 6% of total cellular enzyme/15 min. The secreted form of the hydroxylase incorporated [35S]sulfate, whereas no significant [35S]sulfate was incorporated into the cellular forms of enzyme. We propose that in addition to the dopamine beta-hydroxylase which is found in catecholamine storage vesicles and released during stimulus-coupled exocytosis, PC12 cells also have a constitutive secretory pathway for dopamine beta-hydroxylase and that the enzyme released by this second pathway is sulfated.     

Subcellular Site of Biosynthesis of the Catecholamine Biosynthetic Enzymes in Bovine Adrenal Medulla

The subcellular site of biosynthesis of the catecholamine biosynthetic enzymes was examined. Free and membrane-bound polysomes were prepared from bovine adrenal medulla and mRNA was isolated from these polysomes. Both were active in directing cell-free translations. Immunoprecipitation of cell-free products with specific antisera localized the biosynthesis of the subunits of tyrosine hydroxylase (TH) (apparent Mr = 61,000) and of phenylethanolamine N-methyltransferase (PNMT) (apparent Mr = 32,000) on free polysomes, compared with biosynthesis of subunits of dopamine beta-hydroxylase (DBH) (apparent Mr = 67,000) on membrane-bound polysomes. Cross-reactivity between translation products was observed. Antibodies for DBH recognized a polypeptide with electrophoretic mobility identical to newly synthesized PNMT. However increasing concentrations of antibodies to DBH recognized at most 1/20 of the PNMT formed. The results of this study show the subcellular distribution of the catecholamine synthesizing enzymes is determined by their site of biosynthesis.     

Effect of monensin on synthesis, post-translational processing, and secretion of dopamine beta-hydroxylase from PC12 pheochromocytoma cells

Monensin was used to ascertain the location in the biosynthetic pathway where the 77,000-Mr membrane-bound subunit form of dopamine beta-hydroxylase is post-translationally converted to the 73,000-Mr soluble form. Treatment with low concentrations of monensin (less than or equal to 50 nM) completely depleted the cells of the norepinephrine and dopamine, had a small effect on protein synthesis, and enhanced post-translational processing of only dopamine beta-hydroxylase which was previously synthesized and presumably packaged into neurosecretory vesicles. At these low concentrations, exit from the Golgi apparatus did not appear to be blocked since stimulated secretion of a group of high molecular weight [35S]methionine-labeled proteins was not inhibited. Treatment with higher concentrations of monensin (200 nM) prevented the secretion of the [35S] methionine-labeled proteins normally released with a secretagogue, and also prevented the secretion of [3H] mannose-labeled proteins including dopamine beta-hydroxylase. Surprisingly, a group of lower molecular weight [35S]methionine-labeled proteins was now released from monensin-treated cells. Treatment with high concentrations of monensin (greater than or equal to 200 nM) appeared to block the secretory pathway prior to the packaging step, probably in the Golgi apparatus. If the proteins were packaged prior to monensin treatment, they were released upon stimulation with secretagogues. Monensin treatment (200 nM) enabled the post-translational processing of newly synthesized dopamine beta-hydroxylase, from the 77,000-Mr to the 73,000-Mr subunit form, to go to completion. The susceptibility of this 73,000-Mr subunit form to endoglycosidase H digestion was unaltered, suggesting that dopamine beta-hydroxylase from monensin-treated cells may have the same high mannose oligosaccharide content as native dopamine beta-hydroxylase. These experiments indicate that the post-translational processing of dopamine beta-hydroxylase occurs in the Golgi apparatus and may continue in immature granules prior to their acidification.     

Endoglycosidase H susceptibility of secreted and intracellular forms of dopamine beta-hydroxylase

In an attempt to characterize the differences between various forms of dopamine beta-hydroxylase (DBH) the endoglycosidase H (endo H) susceptibilities of the intracellular and secreted DBH were compared. Both soluble and membrane-bound forms of newly synthesized DBH in PC12 cells were found to be susceptible to endo H. Soluble DBH, apparent Mr of 73K, was converted to 67K Mr and membrane-bound DBH, apparent Mr of 77K and 73K, shifted to 69K and 67K. Upon stimulation with the potent secretagogue, BaCl2, over 65% of intracellular dopamine and norepinephrine were secreted together with the soluble proteins of the secretory granules. DBH, released upon this stimulated secretion, was relatively resistant to endo H compared to the intracellular forms.     

Differences in the incorporation of [3H]-glucosamine into nascent polypeptide chains on free polysomes and two fractions of membrane-bound polysomes in mouse myeloma cells

The incorporation of [³H]-glucosamine into polypeptides of three fractions of polysomes in MPC-11 cells was studied. After short term incubation greatest incorporation was observed in a fraction of membrane-bound polysomes, which after nitrogen cavitation of cells, remained bound to the endoplasmic reticulum (ER) associated with the nucleus (fraction 2). Polypeptide chains on membrane-bound polysomes in the microsomal fraction (fraction 1) and free polysomes contained much less radioactivity. Since nascent polypeptide chains contained within membrane-bound polysomes of fraction 2 are glycosylated at an earlier stage than those in fraction 1 it is likely that this represents a difference in type of proteins synthesized in the respective fractions of ER.     

Initial events involved in the synthesis of the lysosomal enzyme alpha-mannosidase in Dictyostelium discoideum

In Dictyostelium discoideum the lysosomal enzyme alpha-mannosidase is initially synthesized in vivo as a 140,000 Mr protein which is subsequently processed into two mature acidic glycoproteins of 60,000 and 58,000 Mr. To investigate the initial events involved in the synthesis of this protein, mRNA isolated from growing cells was translated in vitro and the resulting protein products were immunoprecipitated with antibodies prepared against the purified enzyme. Messenger RNA prepared from membrane-bound but not free polysomes directed the synthesis of an immunoprecipitable 120K protein that was identified as the alpha-mannosidase primary translation product by a variety of criteria. Translation in vitro in the presence of dog pancreas microsomes resulted in the conversion of the 120K primary translation product to a 140K form. This 140K species was not accessible to added trypsin under conditions preserving membrane integrity, suggesting it is sequestered in the lumen of the endoplasmic reticulum following synthesis. Treatment of either the in vitro modified or cellular 140K alpha-mannosidase precursors with endoglycosidase H resulted in the appearance of proteins 2K larger than the primary translation product. The pulse-labeled cellular precursor and the in vitro processed form have similar isoelectric points as revealed by two-dimensional gel electrophoresis. These results imply that the precursor is N-glycosylated in the endoplasmic reticulum possibly without removal of the signal sequence and that the majority of acidic modifications are added late in the post-translational pathway.     

Soluble and membrane-bound forms of dopamine beta-hydroxylase are encoded by the same mRNA.

A full length cDNA clone for bovine dopamine beta-hydroxylase was expressed in rat pheochromocytoma PC12 cells by stable transformation of this cell line with a plasmid expression vector. The recombinant protein exhibited dopamine beta-hydroxylase enzyme activity and was found in both the soluble and membrane fractions of the secretory vesicle. Immunoprecipitation of cell extracts from recombinant cell lines with dopamine beta-hydroxylase antisera followed by fractionation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two subunits, which migrated to relative molecular masses of 76 and 78 kDa. The recombinant protein co-fractionated with neurotransmitter when subcellular structures were separated by sucrose gradient density centrifugation, suggesting that the protein was routed to the secretory vesicles. Dopamine beta-hydroxylase immunoreactivity in those sucrose gradient fractions presumed to contain secretory vesicles was resistant to treatment with trypsin unless the nonionic detergent Triton X-100 was also present to disrupt membrane structure. The 76- and 78-kDa isoform were each found in both the membrane and soluble fractions of the secretory vesicle. Treatment of cultured cells with nerve growth factor or 8-(4-chlorophenylthio)-cyclic AMP alters the relative distribution of the subunits such that the 76-kDa form predominates. The subcellular distribution of a dopamine beta-hydroxylase cDNA clone lacking the first 16 nucleotide residues was also determined. The predicted amino acid sequence of the protein encoded by this cDNA would be deleted of the first 13 residues of the signal sequence, which were reported to be present in the membrane-bound form, but not the soluble form, of native dopamine beta-hydroxylase (Taljanidisz, J., Stewart, L., Smith, A. J., and Klinman, J. P. (1989) Biochemistry 28, 10054-10061). Immunoprecipitable dopamine beta-hydroxylase derived from expression of the deleted cDNA was found in both the membrane-bound and soluble fractions of the secretory vesicle. These experiments demonstrate that the membrane-bound and soluble forms of dopamine beta-hydroxylase are derived from one primary translation product, which is also sufficient to produce enzyme activity. In addition, the amino-terminal amino acids encoding residues 1-13, which compose the hydrophilic region of the signal sequence, are not necessary for the biogenesis of membrane-bound dopamine beta-hydroxylase.     

Characterization and biosynthesis of soluble and membrane-bound carbonic anhydrase in brain

Carbonic anhydrase from both the cytoplasmic and membrane fractions of the forebrains of rats was characterized with respect to enzymatic activity, immunoreactivity, and in vitro biosynthesis. A procedure for the rapid purification of both membrane-bound and soluble brain carbonic anhydrase is presented that permits retention of full enzymatic activity. Both forms of the enzyme were found to show specific activities of approximately 5500 Units/mg protein when CO2 hydrating activity was determined. In addition, they exhibited similar esterase activity when assayed with p-nitrophenyl acetate. The membrane-bound form, although requiring detergent for extraction from membranes, was freely soluble in aqueous buffers after purification. The molecular weights of both soluble and membrane-bound carbonic anhydrase are 30,000 daltons, and mixing experiments failed to show any significant differences with respect to size. The two forms also exhibit isoelectric points of 7.2. However, the two proteins were found to differ in two respects. Complement fixation indicated that antibodies to soluble carbonic anhydrase had a higher affinity for the soluble form than for the membrane-bound form. The failure to observe any precursor-product relationship between these two proteins with pulse chase studies and the establishment that carbonic anhydrase-like proteins are synthesized on both free polysomes and the rough endoplasmic reticulum indicated that these proteins are synthesized by two separate mechanisms. In vitro synthesis on both free and bound polysomes was determined by two independent methods using different antibodies and different analytical procedures. The basis for these findings and their physiologic importance are discussed.     

Biosynthesis of alpha-fetoprotein in cultured hepatoma cells

Pulse and pulse-chase experiments demonstrated that a heterogeneous polypeptide with an apparent Mr = 68,000 was the first intracellular anti-alpha-fetoprotein (AFP)-precipitable polypeptide synthesized by rat Mc-A-RH-7777 hepatoma cells. The 68,000-dalton polypeptide may consist of polypeptides with apparent molecular weights ranging from 68,000 to 70,000. It was the precursor of two intracellular anti-AFP-precipitable polypeptides of 69,000 and 73,000 apparent molecular weight. The latter were secreted into the medium without further processing. The anti-AFP-precipitable polypeptides in both cells and medium incorporated [3H]glucosamine, indicating that these polypeptides are at least partially glycosylated. The 68,000-dalton polypeptide in cells was bound mostly to concanavalin A-Sepharose, whereas the 69,000-dalton polypeptide was entirely unbound. The 73,000-dalton polypeptide consisted of concanavalin A-bound and -unbound variants. Tunicamycin completely abolished the uptake of [3H]glucosamine into anti-AFT-precipitable polypeptides in both cells and medium, and the resulting polypeptide of apparent Mr = 66,000 did not bind to concanavalin A-Sepharose. Tunicamycin did not affect the synthesis or secretion of AFP by hepatoma cells.     

Characterization of the integral membrane polypeptides of rat liver peroxisomes isolated from untreated and clofibrate-treated rats.

We have characterized the integral membrane polypeptides of liver peroxisomes from untreated rats and rats treated with clofibrate, a peroxisome proliferator. Membranes, prepared by treatment of purified peroxisomes with sodium carbonate, were used to raise an antiserum in rabbits. Immunoblot analysis demonstrated the reaction of this antiserum with six peroxisomal integral membrane polypeptides (molecular masses, 140, 69, 50, 36, 22, and 15 kDa). Treatment of rats with the hypolipidemic drug clofibrate caused a 4- to 10-fold induction in the 69-kDa integral membrane polypeptide, while the other integral membrane polypeptides remained unchanged or varied to a lesser extent. The anti-peroxisomal membrane serum reacted with two integral membrane polypeptides of the endoplasmic reticulum which co-migrated with the 50- and 36-kDa integral membrane polypeptides of the peroxisome. Biochemical and immunoblot analyses indicated that these integral membrane polypeptides were co-localized to peroxisomes and endoplasmic reticulum. Immunoprecipitation of in vitro translation products of RNA isolated from free and membrane-bound polysomes indicated that the 22-, 36-, and 69-kDa integral membrane polypeptides were synthesized on free polysomes, while the 50-kDa integral membrane polypeptide was predominantly synthesized on membrane-bound polysomes. The predominant synthesis of the 50-kDa integral membrane polypeptide on membrane-bound polysomes raises interesting possibilities concerning its biosynthesis.     

Isolation and reconstitution of the membrane-bound form of dopamine beta-hydroxylase

Dopamine beta-hydroxylase is present in the bovine adrenal medulla in two forms, soluble and membrane bound. Previous isolation procedures for the membranous hydroxylase have resulted in a form of enzyme identical in subunit structure with the soluble type. We report here the isolation of a membrane-bound form of dopamine beta-hydroxylase which is structurally different from the soluble form. The isolated membranous enzyme has a large apparent molecular weight on gel filtration, is amphiphilic, and contains bound phospholipid which is predominantly phosphatidylserine. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate shows that the membranous hydroxylase contains two nonidentical subunits under both reducing and nonreducing conditions. Under reducing conditions the apparent molecular weights of the two subunits are 70,000 and 75,000 and both contain carbohydrate. The purified membranous hydroxylase binds to phospholipid vesicles and chymotryptic digestion of the bound enzyme suggests that two forms of the membranous hydroxylase exist.     

Biosynthesis of rat liver pI-6.1 esterase, a carboxylesterase of the cisternal space of the endoplasmic reticulum.

Biosynthesis of the rat liver microsomal esterase with pI 6.1 was investigated in cell-free systems and in cultured hepatocytes, by using a rabbit antiserum. Protein synthesis directed by total rat liver RNA in wheatgerm extract or reticulocyte lysate generated a single immunoprecipitable product, also found with the RNA extracted from bound, but not from free, polysomes. When dog pancreas microsomal fractions were included, reticulocyte lysates gave two processed products, a prominent one slightly larger, and another slightly smaller, than the precursor, both resistant to exogenous proteinases and, hence, segregated within vesicles. The processing was co-translational; it consisted of the removal of a peptide fragment and, for the large component, the addition of a single oligosaccharide chain. Indeed, this component bound to concanavalin A-Sepharose and gave the small one (approximately 2000 Mr loss) by cleavage with endo-beta-N-acetylglucosaminidase H (endo-H). A single labelled peptide was precipitated from hepatocytes incubated with [35S]methionine. Its apparent Mr was decreased by approximately 2000 after treatment with endo-H; it was then identical with that of an unglycosylated form produced in hepatocytes poisoned with tunicamycin. Even in that case, immunoreactive peptides were not detected in the culture medium. Whether synthesized in reticulocyte lysate or in hepatocytes, the glycosylated forms migrated in SDS/polyacrylamide-gel electrophoresis as the purified enzyme labelled with [3H]di-isopropyl fluorophosphate. Thus, although pI-6.1 esterase is not secreted, its biosynthesis is, as yet, indistinguishable from that of secretory proteins. Its oligosaccharide moiety is apparently not the structural element that retains it in the endoplasmic reticulum.     

Synthesis,subcellular distribution and turnover of dopamine beta-hydroxylase in organ cultures of sympathetic ganglia and adrenal medullae

The synthesis, subcellular distribution and turnover of dopamine beta-hydroxylase was studied in organ cultures of rat adrenal medullae and superior cervical ganglia. After exposure to [3H]leucine for 1 or 3 h, the tissues were homogenized at various time intervals and the amount of labelled dopamine beta-hydroxylase in different subcellular fractions (cytosol, soluble and membrane-bound fraction of catecholamine storage vesicles) was determined by immunoprecipitation and subsequent electrophoresis. In cultured adrenal medullae, induction of dopamine beta-hydroxylase initiated in vivo by administration of reserpine affected both soluble and membrane-bound pools of dopamine beta-hydroxylase to a similar extent after pulse-labelling for 1 or 3 h. The half-lives of dopamine beta-hydroxylase, which amounted to 6 h for the cytosol, 7.5 h for the soluble vesicular and 32 h for the membrane-bound vesicular pools were not altered by pretreatment with reserpine. In superior cervical ganglia the half-lives of the soluble pools were 2-3 times longer than in the adrenal medulla, whereas the half-life of the membrane-bound fraction was the same as in the adrenal medulla. In both organs the most heavily labelled fraction (both after a pulse of 1 or 3 h) was always that of the vesicular membrane, suggesting that newly-synthesized dopamine beta-hydroxylase is immediately incorporated into the storage vesicles and not via release into the cytosol from the site of synthesis. The fact that the half-life of membrane-bound dopamine beta-hydroxylase is markedly longer than that of the two soluble pools suggests that the single pools are not only independently supplied by newly-synthesized DBH but there is also no appreciable subsequent exchange between soluble and membrane-bound pools.     

Synthesis and processing of sphingolipid activator protein-2 (SAP-2) in cultured human fibroblasts

Sphingolipid activator proteins (SAP) are relatively small molecular weight proteins that stimulate the enzymatic hydrolysis of sphingolipids in the presence of specific lysosomal hydrolases. SAP-2 has previously been demonstrated to activate the hydrolysis of glucosylceramide, galactosylceramide, and, possibly, sphingomyelin. Using monospecific rabbit antibodies against human spleen SAP-2, the synthesis and processing of SAP-2 were studied in cultured human fibroblasts. When [35S]methionine was presented in the medium to control human cells for 4 h, five major areas of radiolabeling were found. These had apparent molecular weights of 73,000, 68,000, 50,000, 12,000, and 9,000. Further studies indicated that the major extracellular product in normal cells given NH4Cl along with the [35S]methionine and in medium from cultures from patients with I cell disease had an apparent molecular weight of 73,000. The Mr = 68,000 and 73,000 species can be converted to a species with an apparent molecular weight of 50,000 by the action of endoglycosidase F. After labeling cells for 1 h followed by a 1-h chase, the Mr = 12,000 and 9,000 species appear. Treatment of the immunoprecipitated mixture with endoglycosidase F resulted in conversion of these species to one band with an apparent molecular weight of 7,600. These studies indicate that this relatively low molecular weight protein is rapidly synthesized from a relatively large molecular weight highly glycosylated precursor.     

Biosynthesis of intestinal microvillar proteins. Processing of aminopeptidase N by microsomal membranes.

The biosynthesis of small-intestinal aminopeptidase N (EC 3.4.11.2) was studied in a cell-free translation system derived from rabbit reticulocytes. When dog pancreatic microsomal fractions were present during translation, most of the aminopeptidase N synthesized was found in a membrane-bound rather than a soluble form, indicating that synthesis of the enzyme takes place on ribosomes attached to the rough endoplasmic reticulum. The microsomal fractions process the Mr-115 000 polypeptide, which is the primary translation product of aminopeptidase N, to a polypeptide of Mr 140 000. This was found to be sensitive to the action of endo-beta-N-acetylglucosaminidase H (EC 3.2.1.96), showing that aminopeptidase N undergoes transmembrane glycosylation during synthesis. The position of the signal sequence in aminopeptidase N was determined by a synchronized translation experiment. It was found that microsomal fractions should be added before about 25% of the polypeptide was synthesized to ensure processing to the high-mannose glycosylated form. This suggests that the signal sequence is situated in the N-terminal part of the aminopeptidase N. The size of the cell-free translation product in the absence of microsomal fractions was found to be similar to that on one of the forms of the enzyme obtained from tunicamycin-treated organ-cultured intestinal explants.     

Role of glucocorticoids and progesterone in the development of rough endoplasmic reticulum involved in casein biosynthesis.

Hydrocortisone acetate injected into pseudopregnant rabbits induced casein synthesis and a parallel accumulation of casein mRNA. These effects were not accompanied by any enrichment of total RNA in the mammary cell. Hydrocortisone acetate did not favour the attachment of polysomes to endoplasmic reticulum. Casein mRNA concentration was enhanced in free and membrane-bound polysomes. After long treatments, the concentration of casein mRNA reached a plateau in membrane bound polysomes whereas it continued to be accumulated in free polysomes, suggesting that a substantial part of casein synthesis is then carried out by free polysomes. Progesterone injected with high doses of prolactin was unable to prevent the stimulatory action of prolactin on the synthesis of casein, the accumulation of casein mRNA and mammary gland growth, as judged by DNA content. By contrast, the increase in the total RNA content of mammary gland was still significantly reduced by progesterone. In addition, progesterone inhibited almost completely the formation of membrane-bound polysomes and the anchorage of casein mRNA to endoplasmic reticulum. From these data, it was concluded that the formation of the endoplasmic reticulum is not a prerequisite for the initiation of casein synthesis. Glucocorticoids do not play a major role in the formation of the endoplasmic reticulum and the Golai apparatus and in the binding of casein synthesizing polysomes to membranes. Progesteronne is capable of inhibiting preferentially and gradually the stimulation of cellular functions requiring the most potent prolactin stimulation.     

The fat cell beta-adrenergic receptor. Purification and characterization of a mammalian beta 1-adrenergic receptor

The beta 1-adrenergic receptor of rat fat cells was effectively solubilized with digitonin and purified by affinity chromatography and steric exclusion high pressure liquid chromatography (HPLC). The purification strategy described permits an approximately 24,000-fold purification of the beta 1-adrenergic receptor of fat cells with an overall recovery of approximately 70%. Purified receptor preparations demonstrate a specific activity for (-) [3H]dihydroalprenolol binding of 12 nmol/mg of protein. The purified receptor was shown to migrate in steric exclusion HPLC as a Mr = 67,000 protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of radioiodinated purified receptor revealed a single, major peptide of Mr = 67,000. The binding of (-) [3H]dihydroalprenolol to purified receptor preparations displayed stereoselectivity and affinities for antagonists similar in nature to the membrane-bound and digitonin-solubilized beta 1-adrenergic receptor. In addition to the Mr = 67,000 component, a Mr = 140,000 form of the receptor was identified in HPLC runs of freshly prepared, affinity chromatographed receptor preparations that had not been frozen. This larger form of the receptor yielded binding activity of Mr = 67,000 on sequential HPLC runs and was shown to contain the Mr = 67,000 peptide. The beta 1-receptor from this mammalian source, composed of a single Mr = 67,000 peptide, is clearly quite distinct from the purified avian beta 1-, amphibian beta 2-, and mammalian beta 2-adrenergic receptors described by others.     

Synthesis of S100 Protein on Free and Membrane-Bound Polysomes of the Rabbit Brain

Abstract: Free and membrane-bound polysomes were isolated from the cerebral hemispheres and cerebellum of the young adult rabbit. The two polysomal populations were translated in an mRNA-dependent cell-free system derived from rabbit reticulocytes. Analysis of the [³⁵S]methionine-labeled translation products on two-dimensional polyacrylamide gels indicated an efficient separation of the two classes of brain polysomes. The relative synthesis of S100 protein by free and membrane- bound polysomes was determined by direct immuno-precipitation of the cell-free translation products in the presence of detergents to reduce nonspecific trapping. Synthesis of S100 protein was found to be twofold greater on membrane-bound polysomes compared with free polysomes isolated from either the cerebral hemispheres or the cerebellum. In addition, the proportion of poly- (A+)mRNA coding for SlOO protein was also twofold greater in membrane-bound polysomes compared with free polysomes isolated from the cerebral hemispheres. These results indicate that the cytoplasmic S100 protein is synthesized predominantly on membrane-bound polysomes in the rabbit brain. We suggest that the nascent S100 polypeptide chain translation complex is attached to the rough endoplasmic reticulum by an ionic interaction involving a sequence of 13 basic amino acids in S100 protein.     

Tissue inhibitor of metalloproteinases. Identification of precursor forms synthesized by human fibroblasts in culture

Precursor forms of the glycoprotein tissue inhibitor of metalloproteinases (TIMP) synthesized by human fibroblasts in culture have been identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of specific immunoprecipitates. Translation of mRNA extracted from fibroblasts in the cell-free rabbit reticulocyte lysate system yielded a single immunoprecipitable precursor of tissue inhibitor of metalloproteinases, Mr 22000. Intact fibroblasts cultured in the presence of tunicamycin synthesized an Mr 20 000 form of tissue inhibitor of metalloproteinases, detectable intracellularly and extracellularly. This is in contrast to the predominantly intracellular Mr 24 000 form synthesized during monensin treatment of cells and the normal secreted form of tissue inhibitor of metalloproteinases, Mr 29 000. Isoelectric focusing of the various immunoprecipitable precursor forms showed a progressive increase in positive charge and microheterogeneity of the protein during cellular processing. The data suggest that the inhibitor protein core, of basic pI, is glycosylated initially by the addition of mostly neutral sugars and subsequently by acidic sugars, prior to secretion.