A previously unrecognized subunit of the receptor for immunoglobulin E

Our laboratory previously found that under conditions that stabilized the interaction between the alpha and beta subunits of the receptor for immunoglobulin E, two new components were recovered having apparent molecular weights of 45 000 and 20 000, respectively. In this paper, we characterize the 20-kDa material. We demonstrate that it consists of a disulfide-linked dimer of 10-kDa polypeptides and that these have all the characteristics expected for subunits of the receptor. We propose that they be termed gamma chains and that the receptor consists of four chains: one alpha, one beta, and two gamma chains. The gamma chains share many of the labeling properties of the beta chain and, like the latter, are likely to be embedded in the plasma membrane and exposed on the internal but not the external surface of the bilayer.     

Identification of exposed and buried determinants of the membrane-bound acetylcholine receptor from Torpedo californica

The ability of five rabbit anti-acetylcholine receptor antisera to recognize the membrane-bound receptor from Torpedo californica has been investigated. Two antisera, raised against affinity-purified native receptor, react extensively with purified receptor-rich membrane vesicles. Since the membrane vesicles are impermeable to macromolecules and are oriented right side out, these two antisera recognize predominantly extracellular determinants. Two antisera against sodium dodecyl sulfate denatured receptor and one against purified delta subunit react poorly with the membrane-bound receptor. Only 10-20% of the determinants recognized by these antisera are accessible to antibodies when the receptor is membrane bound. Many of the latent sites can be exposed by permeabilizing the vesicles with saponin, by alkaline extraction of the membranes to remove peripheral proteins, or by a combination of these two treatments. These treatments neither solubilize the receptors nor interfere with their ability to undergo agonist-induced affinity changes. Subunit analysis of the sites on the membrane-bound receptor that are accessible to antibodies indicates that the alpha, beta, and delta chains possess extracellular determinants. Buried sites are present on all four of the subunits. Saponin permeabilization makes latent sites accessible on alpha and delta while alkaline extraction uncovers determinants on alpha, gamma, and delta. Treatment of membranes by both procedures reveals sites on beta, gamma, and delta that are not uncovered by either treatment alone. This study, in conjunction with results from other laboratories demonstrating that the gamma chain is extracellularly exposed, suggests that all four subunits are transmembrane proteins.     

The purified alpha subunits of Go and Gi from bovine brain require beta gamma for association with phospholipid vesicles

The purified G-proteins from bovine brain were examined for potential solubility in the absence of detergent. The isolated alpha o and alpha i subunits migrated through sucrose with rates consistent with the existence of monomeric species either in the presence or the absence of cholate. The beta gamma subunits or holo-G-proteins aggregated extensively if cholate was absent. Al3+, Mg2+, and F- prevented the aggregation of alpha o and alpha i caused by the addition of beta gamma and could also prevent the aggregation of alpha s when Gs was examined at higher temperature. The association of subunits with phospholipid vesicles was examined. Whereas beta gamma associated totally with phospholipid vesicles, purified alpha o showed little interaction. alpha o did bind to vesicles containing beta gamma (beta gamma vesicles) in a saturable fashion that indicated a stoichiometric association between the subunits. Treatment with guanosine 5'-(3-O-thio)triphosphate could partially dissociate alpha o that was bound to beta gamma vesicles. These data suggest that beta gamma may be an anchor for association of alpha subunits with membranes and that regulation by these proteins may not be limited to the plasma membrane. This possibility and its implications are discussed. The reversible association of alpha o to beta gamma vesicles may provide a very sensitive system for the study of the interactions between these subunits.     

Internalization of insulin receptors in the isolated rat adipose cell. Demonstration of the vectorial disposition of receptor subunits

The internalization of the insulin receptor in the isolated rat adipose cell and the spatial orientation of the alpha (Mr = 135,000) and beta (Mr = 95,000) subunits of the receptor in the plasma membrane have been examined. The receptor subunits were labeled by lactoperoxidase/Na125I iodination, a technique which side-specifically labels membrane proteins in intact cells and impermeable membrane vesicles. Internalization was induced by incubating cells for 30 min at 37 degrees C in the presence of saturating insulin. Plasma, high density microsomal (endoplasmic reticulum-enriched), and low density microsomal (Golgi-enriched) membrane fractions were prepared by differential ultracentrifugation. Receptor subunit iodination was analyzed by immunoprecipitation with anti-receptor antibodies, sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and autoradiography. When intact cells were surface-labeled and incubated in the absence of insulin, the alpha and beta receptor subunits were clearly observed in the plasma membrane fraction and their quantities in the microsomal membrane fractions paralleled plasma membrane contamination. Following receptor internalization, however, both subunits were decreased in the plasma membrane fraction by 20-30% and concomitantly and stoichiometrically increased in the high and low density microsomal membrane fractions, without alterations in either their apparent molecular size or proportion. In contrast, when the isolated particulate membrane fractions were directly iodinated, both subunits were labeled in the plasma membrane fraction whereas only the beta subunit was prominently labeled in the two microsomal membrane fractions. Iodination of the subcellular fractions following their solubilization in Triton X-100 again clearly labeled both subunits in all three membrane fractions in identical proportions. These results suggest that 1) insulin receptor internalization comprises the translocation of both major receptor subunits from the plasma membrane into at least two different intracellular membrane compartments associated, respectively, with the endoplasmic reticulum and Golgi-enriched membrane fractions, 2) this translocation occurs without receptor loss or alterations in receptor subunit structure, and 3) the alpha receptor subunit is primarily, if not exclusively, exposed on the extracellular surface of the plasma membrane while the beta receptor subunit traverses the membrane, and this vectorial disposition is inverted during internalization.     

Transmembrane topography of nicotinic acetylcholine receptor: immunochemical tests contradict theoretical predictions based on hydrophobicity profiles

In our preceding paper [Ratnam, M., Sargent, P. B., Sarin, V., Fox, J. L., Le Nguyen, D., Rivier, J., Criado, M., & Lindstrom, J. (1986) Biochemistry (preceding paper in this issue)], we presented results from peptide mapping studies of purified subunits of the Torpedo acetylcholine receptor which suggested that the sequence beta 429-441 is on the cytoplasmic surface of the receptor. Since this finding contradicts earlier theoretical models of the transmembrane structure of the receptor, which placed this sequence of the beta subunit on the extracellular surface, we investigated the location of the corresponding sequence (389-408) and adjacent sequences of the alpha subunit by a more direct approach. We synthesized peptides including the sequences alpha 330-346, alpha 349-364, alpha 360-378, alpha 379-385, and alpha 389-408 and shorter parts of these peptides. These peptides corresponded to a highly immunogenic region, and by using 125I-labeled peptides as antigens, we were able to detect in our library of monoclonal antibodies to alpha subunits between two and six which bound specifically to each of these peptides, except alpha 389-408. We obtained antibodies specific for alpha 389-408 both from antisera against the denatured alpha subunit and from antisera made against the peptide. These antibodies were specific to alpha 389-396. In binding assays, antibodies specific for all of these five peptides bound to receptor-rich membrane vesicles only after permeabilization of the vesicles to permit access of the antibodies to the cytoplasmic surface of the receptors, suggesting that the receptor sequences which bound these antibodies were located on the intracellular side of the membrane. Electron microscopy using colloidal gold to visualize the bound antibodies was used to conclusively demonstrate that all of these sequences are exposed on the cytoplasmic surface of the receptor. These results, along with our previous demonstration that the C-terminal 10 amino acids of each subunit are exposed on the cytoplasmic surface, show that the hydrophobic domain M4 (alpha 409-426), previously predicted from hydropathy profiles to be transmembranous, does not, in fact, cross the membrane. Further, these results show that the putative amphipathic transmembrane domain M5 (alpha 364-399) also does not cross the membrane. Our results thus indicate that the transmembrane topology of a membrane protein cannot be deduced strictly from the hydropathy profile of its primary amino acid sequence. We present a model for the transmembrane orientation of receptor subunit polypeptide chains which is consistent with current data.     

Topological analysis of the pyridine nucleotide transhydrogenase of Escherichia coli using proteolytic enzymes

The pyridine nucleotide transhydrogenase of Escherichia coli has an alpha 2 beta 2 structure (alpha: Mr, 54,000; beta: Mr, 48,700). Hydropathy analysis of the amino acid sequences suggested that the 10 kDa C-terminal portion of the alpha subunit and the N-terminal 20-25 kDa region of the beta subunit are composed of transmembranous alpha-helices. The topology of these subunits in the membrane was investigated using proteolytic enzymes. Trypsin digestion of everted cytoplasmic membrane vesicles released a 43 kDa polypeptide from the alpha subunit. The beta subunit was not susceptible to trypsin digestion. However, it was digested by proteinase K in everted vesicles. Both alpha and beta subunits were not attacked by trypsin and proteinase K in right-side out membrane vesicles. The beta subunit in the solubilized enzyme was only susceptible to digestion by trypsin if the substrates NADP(H) were present. NAD(H) did not affect digestion of the beta subunit. Digestion of the beta subunit of the membrane-bound enzyme by trypsin was not induced by NADP(H) unless the membranes had been previously stripped of extrinsic proteins by detergent. It is concluded that binding of NADP(H) induces a conformational change in the transhydrogenase. The location of the trypsin cleavage sites in the sequences of the alpha and beta subunits were determined by N- and C-terminal sequencing. A model is proposed in which the N-terminal 43 kDa region of the alpha subunit and the C-terminal 30 kDa region of the beta subunit are exposed on the cytoplasmic side of the inner membrane of E. coli. Binding sites for pyridine nucleotide coenzymes in these regions were suggested by affinity chromatography on NAD-agarose columns.     

Monoclonal antibodies to cytoplasmic domains of the acetylcholine receptor

Fourteen clonal hybridoma lines that secrete monoclonal antibodies (mabs) to the Torpedo acetylcholine receptor (AChR) have been isolated. When analyzed by an immunoreplica technique, two mabs recognized the alpha subunit, three reacted with the beta subunit, one reacted with the gamma chain, and five recognized the delta subunit. One mab failed to react with any of the subunits using this assay and two mabs recognized determinants found on both the gamma and the delta subunits. These were classified according to their reactivities with the membrane-bound Torpedo AChR. One category is comprised of mabs (including both anti-alpha mabs) that recognize extracellular epitopes. A second classification included mabs that are unable to bind the membrane-associated AChR. The third category is comprised of mabs directed against cytoplasmic epitopes of the AChR. The latter mabs, all of which recognize the gamma or delta subunits or both, bind only slightly to sealed, outside-out Torpedo vesicles. The binding is increased 10-20-fold by either alkaline extraction or treatment of the vesicles with 10 mM lithium diiodosalicylate but not by permeabilization of the vesicles with saponin. Three of the six mabs in this category react with frog muscle endplates but only if the cytoplasmic surface of the membrane is accessible.     

Structural arrangement and conformational dynamics of the gamma subunit of the Na+/K+-ATPase

The Na+/K+-ATPase couples the chemical energy in ATP to transport Na+ and K+ across the plasma membrane against a concentration gradient. The ion pump is composed of two mandatory subunits: the alpha subunit, which is the major catalytic subunit, and the beta subunit, which is required for proper trafficking of the complex to the plasma membrane. In some tissues, the ion pump also contains an optional third subunit, gamma, which modulates the pump activity. To examine the conformational dynamics of the gamma subunit during ion transport and its position in relation to the alpha and the beta subunits, we have used fluorescence resonance energy transfer under voltage clamp conditions. From these experiments, evidence is provided that the gamma subunit is located adjacent to the M2-M6-M9 pocket of the alpha subunit at the transmembrane-extracellular interface. We have also used fluorescence resonance energy transfer to investigate the relative movement of the three subunits as the ion pump shuttles between the two main conformational states, E1 and E2, as described by the Albers-Post scheme. The results from this study suggest that there is no relative change in distance between the alpha and gamma subunits but there is a relative change in distance between the beta and gamma subunits during the E2 to E1 transition. It was also observed that labeling the gamma subunit at specific residues with fluorophores induces a decrease in K+-induced stationary current. This result could be due to a perturbation in the K+ branch of the reaction cycle of the pump, representing a new way to inhibit the pump.     

Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Mutation of Galpha(q) or Galpha(s) N-terminal contact sites for Gbetagamma resulted in alpha subunits that failed to localize at the plasma membrane or undergo palmitoylation when expressed in HEK293 cells. We now show that overexpression of specific betagamma subunits can recover plasma membrane localization and palmitoylation of the betagamma-binding-deficient mutants of alpha(s) or alpha(q). Thus, the betagamma-binding-defective alpha is completely dependent on co-expression of exogenous betagamma for proper membrane localization. In this report, we examined the ability of beta(1-5) in combination with gamma(2) or gamma(3) to promote proper localization and palmitoylation of mutant alpha(s) or alpha(q). Immunofluorescence localization, cellular fractionation, and palmitate labeling revealed distinct subtype-specific differences in betagamma interactions with alpha subunits. These studies demonstrate that 1) alpha and betagamma reciprocally promote the plasma membrane targeting of the other subunit; 2) beta(5), when co-expressed with gamma(2) or gamma(3), fails to localize to the plasma membrane or promote plasma membrane localization of mutant alpha(s) or alpha(q); 3) beta(3) is deficient in promoting plasma membrane localization of mutant alpha(s) and alpha(q), whereas beta(4) is deficient in promoting plasma membrane localization of mutant alpha(q); 4) both palmitoylation and interactions with betagamma are required for plasma membrane localization of alpha.     

Treatments that extract the 43K protein from acetylcholine receptor clusters modify the conformation of cytoplasmic domains of all subunits of the receptor.

The nicotinic acetylcholine receptor (AChR) of Torpedo electric organ and vertebrate skeletal muscle is closely associated with a Mr 43,000 protein (43K). In this study, we have examined the effects on the AChR of treatments which remove the 43K protein. We used semiquantitative fluorescence techniques to measure the binding of antibodies to clustered AChR in cultured rat myotubes. We found that labeling by antibodies to the cytoplasmic portions of each of the four receptor polypeptides increased significantly upon extraction of the 43K protein. Labeling by an antibody to an extracellular epitope of the alpha subunits was not affected by removal of the 43K protein, suggesting that changes were restricted to the cytoplasmic domains of the AChR. Increases in labeling by antibodies were more limited following protease treatment, which removes most cytoskeletal structures but leaves the 43K protein bound to the membrane. Competition between an antibody to the beta subunit and an antibody to the gamma and delta subunits suggests that the cytoplasmic portion of the AChR still retains a degree of native structure in the absence of the 43K protein. Our results suggest that, although some of these changes may be due to simply exposing additional epitopes on the AChR, the cytoplasmic portions of all the subunits of the AChR undergo significant conformational changes upon extraction of the 43K protein.     

Regulation of signal transfer from beta 1-adrenoceptor to adenylate cyclase by beta gamma subunits in a reconstituted system

The beta gamma subunits of guanine nucleotide binding proteins from bovine brain and bovine rod outer segments have different structural and immunochemical properties. In spite of these structural differences, beta gamma subunits from these sources have been found to be fully interchangeable in terms of their interaction with alpha subunits of pertussis-toxin-sensitive G proteins. In contrast, however, there are striking differences between these beta gamma subunits with regard to their ability to deactivate fluoride-stimulated Gs. These profound differences were also observed when the interaction of the purified components of the adenylate cyclase system was studied after reconstitution into phospholipid vesicles. Addition of beta gamma purified from bovine brain to vesicles containing beta-receptor and Gs results in a biphasic effect on receptor-stimulated GTPase activity, whereas addition of transducin beta gamma was virtually without any effect. Likewise, beta gamma from bovine brain, but not transducin beta gamma, affected adenylate cyclase activity of a reconstituted system consisting of three purified components (R, Gs, C). Thus, the alpha subunit of Gs, but not the alpha subunits of pertussis-toxin-sensitive G proteins discriminate between structurally different beta gamma subunits.     

Hydrophobic properties of the beta 1 and beta 2 subunits of the rat brain sodium channel

Voltage-sensitive sodium channels purified from rat brain in functional form consist of a stoichiometric complex of three glycoprotein subunits, alpha of 260 kDa, beta 1 of 36 kDa, and beta 2 of 33 kDa. The alpha and beta 2 subunits are linked by disulfide bonds. The hydrophobic properties of these three subunits were examined by covalent labeling with the photoreactive hydrophobic probe 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine [( 125I]TID) which labels transmembrane segments in integral membrane proteins. All three subunits of the sodium channel were labeled by [125I]TID when the purified protein was solubilized in mixed micelles of Triton X-100 and phosphatidylcholine (4:1). The half-time for photolabeling was approximately 7 min consistent with the half-time of 9 min for photolysis of TID under our conditions. Comparable amounts of TID per mg of protein were incorporated into each subunit. Purified sodium channels reconstituted in phosphatidylcholine vesicles were also labeled by TID with comparable incorporation per mg of protein into all three subunits. The efficiency of photolabeling of the three subunits was reduced from 39 to 44% by a 2-fold expansion of the hydrophobic phase of the reaction mixture but was unaffected by a 2-fold expansion of the aqueous phase, confirming that the photolabeling reaction took place in the lipid phase of the vesicle bilayer. The hydrophobic properties of the sodium channel subunits were examined further using phase separation in the nonionic detergent Triton X-114. Under conditions in which beta 1 is dissociated from alpha, the beta 1 subunit was preferentially extracted into the Triton X-114 phase, and the disulfide-linked alpha beta 2 complex was retained in the aqueous phase. When the disulfide bonds between the alpha and beta 2 subunits were reduced with dithioerythritol, the beta 2 subunit was also preferentially extracted into the Triton X-100 phase leaving the free alpha subunit in the aqueous phase. A preparative method for isolation of the beta 1 and beta 2 subunits was developed based on this technique. Considered together, the results of our hydrophobic labeling and phase separation experiments indicate that the alpha, beta 1, and beta 2 subunits all have substantial hydrophobic domains that may interact with the hydrocarbon phase of phospholipid bilayer membranes. Since the alpha subunit is known to be a transmembrane protein with many potential membrane-spanning segments, we conclude that the beta 1 and beta 2 subunits are likely to also be integral membrane proteins with one or more membrane-spanning segments.(ABSTRACT TRUNCATED AT 400 WORDS)     

Subcellular localization and function of alternatively spliced Noxo1 isoforms

Nox organizer 1 (Noxo1), a p47(phox) homolog, is produced as four isoforms with unique N-terminal PX domains derived by alternative mRNA splicing. We compared the subcellular distribution of these isoforms or their isolated PX domains produced as GFP fusion proteins, as well as their ability to support Nox1 activity in several transfected models. Noxo1alpha, beta, gamma, and delta show different subcellular localization patterns, determined by their PX domains. In HEK293 cells, Noxo1beta exhibits prominent plasma membrane binding, Noxo1gamma shows plasma membrane and nuclear associations, and Noxo1alpha and delta localize primarily on intracellular vesicles or cytoplasmic aggregates, but not the plasma membrane. Nox1 activity correlates with Noxo1 plasma membrane binding in HEK293 cells, since Noxo1beta supports the highest activity and Noxo1gamma and Noxo1alpha support moderate or low activities, respectively. In COS-7 cells, where Noxo1alpha localizes on the plasma membrane, the activities supported by the three isoforms (alpha, beta, and gamma) do not differ significantly. The PX domains of beta and gamma bind the same phospholipids, including phosphatidic acid. These results indicate that the variant PX domains are unique determinants of Noxo1 localization and Nox1 function. Finally, the overexpressed Noxo1 isoforms do not affect p22(phox) localization, although Nox1 is needed to transport p22(phox) to the plasma membrane.     

Assembly and clustering of acetylcholine receptors containing GFP-tagged epsilon or gamma subunits: selective targeting to the neuromuscular junction in vivo.

Acetylcholine receptor (AChR) gamma and epsilon subunits were tagged by green fluorescent protein (GFP) to analyse assembly and targeting in live muscle fibers at the neuromuscular junction. N- or C-terminal fusion polypeptides showed no fluorescence upon transfection of HEK cells. When GFP was inserted into the cytoplasmic loop connecting putative transmembrane regions M3 and M4, the gamma/GFP and epsilon/GFP subunits were fluorescent and formed together with the alpha, beta, and delta subunits GFP-tagged AChR complexes that were integrated into the plasma membrane. As the AChR were also clustered by rapsyn, the results indicate that the cytoplasmatic domains of the gamma and epsilon subunits may not be required for assembly and rapsyn-dependent clustering. The gamma/GFP and epsilon/GFP subunit-containing receptors were expressed in X. laevis oocytes and have affinities for acetylcholine similar to that of the wild-type receptors. Direct gene transfer into single muscle fibers reveals that gamma/GFP or epsilon/GFP polypeptides are expressed at the site of injection and are transported within the endoplasmatic reticulum. When reaching subsynaptic regions, both gamma/GFP or epsilon/GFP subunits compete with endogenous epsilon subunits to assemble GFP-tagged receptors, which are selectively targeted to the postsynaptic membrane.     

The subunit structure of the insulin receptor and molecular interactions with major histocompatibility complex antigens

Insulin receptors were labeled with 125I-photoreactive insulin (specifically labeling alpha-subunits) and by insulin-stimulated autophosphorylation (specifically labeling beta-subunits). The results show that the insulin receptor exists under different free and disulfide-linked combinations of alpha and beta subunits. Moreover, the insulin receptor is closely associated to class I antigens of the major histocompatibility complex to form a high molecular weight multi-molecular membrane complex.     

GABAA receptor subtypes immunopurified from rat brain with alpha subunit-specific antibodies have unique pharmacological properties.

The unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor were expressed in bacterial and used to produce subunit-specific polyclonal antisera. Antibodies immobilized on protein A-Sepharose were used to isolate naturally occurring alpha-specific populations of GABAA receptors from rat brain that retained the ability to bind [3H]muscimol, [3H]flunitrazepam, [3H]Ro15-1788, and [125I]iodo-clonazepam with high affinity. Pharmacological characterization of these subtypes revealed marked differences between the isolated receptor populations and was generally in agreement with the reported pharmacological profiles of GABAA receptors in cells transiently transfected with alpha 1 beta 1 gamma 2, alpha 2 beta 1 gamma 2, alpha 3 beta 1 gamma 2, and alpha 5 beta 1 gamma 2 combinations of subunits. Additional subtypes were also identified that bind [3H]muscimol but do not bind benzodiazepines with high affinity. The majority of GABAA receptor oligomers contains only a single type of alpha subunit, and we conclude that alpha 1, alpha 2, alpha 3, and alpha 5 subunits exist in vivo in combination with the beta subunit and gamma 2 subunit.     

Topological dispositions of lysine alpha 380 and lysine gamma 486 in the acetylcholine receptor from Torpedo californica

The locations have been determined, with respect to the plasma membrane, of lysine alpha 380 and lysine gamma 486 in the alpha subunit and the gamma subunit, respectively, of the nicotinic acetylcholine receptor from Torpedo californica. Immunoadsorbents were constructed that recognize the carboxy terminus of the peptide GVKYIAE released by proteolytic digestion from positions 378-384 in the amino acid sequence of the alpha subunit of the acetylcholine receptor and the carboxy terminus of the peptide KYVP released by proteolytic digestion from positions 486-489 in the amino acid sequence of the gamma subunit. They were used to isolate these peptides from proteolytic digests of polypeptides from the acetylcholine receptor. Sealed vesicles containing the native acetylcholine receptor were labeled with pyridoxal phosphate and sodium [3H]-borohydride. Saponin was added to a portion of the vesicles prior to labeling to render them permeable to pyridoxal phosphate. The effect of saponin on the incorporation of pyridoxamine phosphate into lysine alpha 380 and lysine gamma 486 from the acetylcholine receptor in these vesicles was assessed with the immunoadsorbents. The peptides bound and released by the immunoadsorbents were positively identified and quantified by high-pressure liquid chromatography. Modification of lysine alpha 380 in the native acetylcholine receptor in sealed vesicles increased 5-fold in the presence of saponin, while modification of lysine gamma 486 was unaffected by the presence of saponin. The conclusions that follow from these results are that lysine alpha 380 is on the inside surface of a vesicle and lysine gamma 486 is on the outside surface.(ABSTRACT TRUNCATED AT 250 WORDS)     

Subunit interaction and function of clathrin-coated vesicle adaptors from the Golgi and the plasma membrane

Clathrin in coated vesicles is linked to transmembrane receptors by adaptor protein complexes. The Golgi-associated adaptor complex HA1 is a tetramer, made up of beta', gamma, 47-kDa, and 20-kDa subunits, whereas the tetrameric plasma membrane adaptor, HA2, contains alpha, beta, 50-kDa, and 16-kDa subunits (Ahle, S., Mann, A., Eichelsbacher, U., and Ungewickell, E. (1988) EMBO J. 7, 919-929). Here we report on the structural organization of adaptor subunits as revealed by proteolytic dissection. We show that the beta' and gamma subunits of HA1 are cleaved into 60-67-kDa "trunk" and 32-44-kDa "head" fragments. Interactions between adaptor subunits involve the trunk domains only. In overall organization of their domains, the Golgi and plasma membrane adaptors are very similar. The similarity encompasses also the location of phosphorylated serine residues in the alpha a, beta, beta', and gamma subunits, which are found in the head domains in all cases. In the alpha a and beta subunits they probably occur in the proline- and glycine-rich hinge region, which connects the head to the trunk. Identical adaptor fragments were obtained by controlled digestion of clathrin-coated vesicles. Under conditions that did not affect the integrity of the clathrin heavy chain, the adaptor head fragments were always quantitatively released from coated vesicles. The release of the bulk of the adaptors occurred concomitantly with the cleavage of their beta-type subunits (beta and beta') and under buffer conditions that prevent aggregation of adaptors. These observations taken together with the results of reconstitution experiments confirm and extend previous data (Ahle, S., and Ungewickell, E. (1989) J. Biol. Chem. 264, 20089-20093) which suggested that adaptors attach to clathrin through their beta-type (beta and beta') subunits. Moreover, high affinity interaction between adaptors and clathrin requires the participation of regions from both the head and trunk domains of the beta-type subunits.     

Assembly and function of integrin receptors is dependent on opposing alpha and beta cytoplasmic domains.

The membrane proximal regions of integrin alpha and beta subunits are highly conserved in evolution. In particular, all integrin alpha subunits share the KXGFFKR sequence at the beginning of their cytoplasmic domains. Previous work has shown that this domain is important in integrin receptor assembly. Using chimeric integrin alpha and beta subunits, we show that the native cytoplasmic domains of both subunits must be present for efficient assembly. Most strikingly, chimeric alpha 1 and beta 1 subunits with reciprocally swapped intracellular domains dimerize selectively into collagen IV receptors expressed at high levels on the surface. However, these receptors, which bind ligand efficiently, are deficient in a variety of post-ligand binding events, including cytoskeletal association and induction of tyrosine phosphorylation. Furthermore, deletion of the distal alpha cytoplasmic domain in the swapped heterodimers leads to ligand-independent focal contact localization, which also occurs in wild-type subunits when the distal cytoplasmic domain is deleted. These results show that proper integrin assembly requires opposed alpha and beta cytoplasmic domains, and this opposition prevents ligand-independent focal contact localization. Our working hypothesis is that these two domains may associate during receptor assembly and provide the mechanism for integrin receptor latency.     

Visualization of G protein betagamma dimers using bimolecular fluorescence complementation demonstrates roles for both beta and gamma in subcellular targeting

To investigate the role of subcellular localization in regulating the specificity of G protein betagamma signaling, we have applied the strategy of bimolecular fluorescence complementation (BiFC) to visualize betagamma dimers in vivo. We fused an amino-terminal yellow fluorescent protein fragment to beta and a carboxyl-terminal yellow fluorescent protein fragment to gamma. When expressed together, these two proteins produced a fluorescent signal in human embryonic kidney 293 cells that was not obtained with either subunit alone. Fluorescence was dependent on betagamma assembly in that it was not obtained using beta2 and gamma1, which do not form a functional dimer. In addition to assembly, BiFC betagamma complexes were functional as demonstrated by more specific plasma membrane labeling than was obtained with individually tagged fluorescent beta and gamma subunits and by their abilities to potentiate activation of adenylyl cyclase by alpha(s) in COS-7 cells. To investigate isoform-dependent targeting specificity, the localization patterns of dimers formed by pair-wise combinations of three different beta subunits with three different gamma subunits were compared. BiFC betagamma complexes containing either beta1 or beta2 localized to the plasma membrane, whereas those containing beta5 accumulated in the cytosol or on intracellular membranes. These results indicate that the beta subunit can direct trafficking of the gamma subunit. Taken together with previous observations, these results show that the G protein alpha, beta, and gamma subunits all play roles in targeting each other. This method of specifically visualizing betagamma dimers will have many applications in sorting out roles for particular betagamma complexes in a wide variety of cell types.