Reconstitution of model membranes from phospholipid and outer membrane proteins of Proteus mirabilis. Role of proteins in the formation of hydrophilic pores and protection of membranes against detergents.

Outer membrane proteins extracted from isolated cell walls of Proteus mirabilis were able to combine the cell wall phospholipids in a model membrane system. The presence of outer membrane proteins in vesicular model membranes mediated the release of previously entrapped [14C]sucrose while [3H]inulin was retained. Incorporation of lipopolysaccharide from the same cell walls was not required for the formation of such selectively permeable membranes. Three major outer membrane proteins of apparent molecular weights 39000, 36000 and 17000 were isolated using acetic acid and sodium deoxycholate solution as solvents and avoiding the strongly denaturing sodium dodecyl sulfate. The isolated proteins were assayed for their ability to form hydrophilic pores in reconstituted membranes. The trypsin-sensitive 39000-Mr protein and the peptidoglycan-associated 36000-Mr protein were equally effective in this function whereas the 17000-Mr protein mediated little penetration of low molecular weight solute. The 39000-Mr and 36000-Mr proteins also protected reconstituted membrane vesicles from disruption by detergent while 17000-Mr protein was ineffective in this regard.     

A Pseudomonas stutzeri outer membrane protein inserts copper into N2O reductase

Among a set of frameshift mutagen (ICR-191; Polysciences, Inc.)-induced mutations that confer inability to grow anaerobically with N2O as the sole electron acceptor, one class was found that produced an inactive N2O reductase which lacked copper. All of these mutant strains failed to produce a 61,000-Mr protein located in the outer membrane. This protein, termed NosA, seems not to be responsible for bringing copper into the cell because the mutant strains and their parent were similarly sensitive to the copper content of the growth medium and no intermediate copper concentration in the medium permitted the mutant strains (nosA) to grow anaerobically with N2O as the sole electron acceptor. We conclude that NosA is necessary to insert copper into N2O reductase or to maintain it there.     

Production of a soluble form of fumarate reductase by multiple gene duplication in Escherichia coli K12.

1. Ampicillin-hyperresistant mutants of Escherichia coli K12 bearing multiple gene duplications in the ampC (beta-lactamase) gene region of the chromosome overproduced at least six proteins with molecular weights 97,000, 80,000, 72,000, 49,000, 33,000 and 26,500 during anaerobic growth. All but two of the proteins (80,000-Mr and 49,000-Mr) were also overproduced during aerobic growth. The distribution of the proteins in soluble and particulate cell fractions was investigated. 2. The 33,000-Mr and 72,000-Mr components were identified as beta-lactamase and the amp-linked frdA gene product, fumarate reductase, respectively. Co-sedimentation of the 26,500-Mr component with the fumarate reductase suggested that the smaller protein could be functionally related to the reductase. The lack of correspondence between the amplified proteins and the products of other amp-linked genes, aspA and mop(groE), indicated that these genes are not included in the repetitive sequence. 3. Fumarate reductase activities were amplified up to 32-fold by the multiple gene duplications. Two forms of fumarate reductase were produced: particulate (membrane-bound) and soluble (cytoplasmic). Production of the soluble form occurred when the binding capacity of the membrane was saturated. Both forms of fumarate reductase were enzymically active but the soluble form was readily inactivated under assay conditions.     

Composition of major outer membrane proteins of Vibrio vulnificus isolates: effect of different growth media and iron deficiency

The outer membrane proteins of Vibrio vulnificus including isolates from humans, seawater and an asari clam were examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. A major outer membrane protein with an apparent molecular weight of 48,000 (48K protein) was common to all the strains grown in 3% NaCl-nutrient broth; however this 48K protein was not produced in any of the strains grown in chemically defined medium. Other major outer membrane proteins with molecular weights ranging from 33,000 to 40,000 varied in number, relative amount and molecular weight depending on the strain. One to three new outer membrane proteins with molecular weights ranging from 74,000 to 85,000 were produced in the cells grown in iron-deficient medium. The 48K protein and one or two major proteins with molecular weights ranging from 35,000 to 37,000 in the cells grown in 3% NaCl-nutrient broth were not solubilized by 2% SDS at 60 C for 30 min and were resistant to trypsin, indicating that they are porins. On the other hand, in cells grown in chemically defined medium, one or two major outer membrane proteins with molecular weights ranging from 33,000 to 40,000 might be porins.     

Expression of the divergent tricarboxylate transport operon (tctI) of Salmonella typhimurium.

Membrane-associated gene products of shock-sensitive bacterial transport operons are often difficult to detect. A 4.5-kilobase DNA fragment, known to completely encode the Salmonella typhimurium tctI operon, was cloned in both orientations behind the T7 phage promoter phi 10 and expressed by using the T7 polymerase-promoter system of Tabor and Richardson (S. Tabor and C. C. Richardson, Proc. Natl. Acad. Sci. USA 82:1074-1078, 1985). Under these conditions, five proteins were clearly demonstrated. One DNA strand was shown to encode the periplasmic (29,000-Mr) C protein (as a 31,000-Mr precursor), a 19,000-Mr protein, and a 40,000- to 45,000-Mr protein which ran as a diffuse band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The opposite strand carried the information for two additional proteins of 29,000 and 14,000 Mr. By Tn5 mutagenesis, subcloning of Tn5 insertions, and subcloning of various deletion mutants it was shown that the tctI system is divergently transcribed. The periplasmic binding protein (C protein) is the first product of one operon, followed by the 19,000-Mr and 45,000-Mr integral inner membrane proteins. On the opposite strand only the 29,000-Mr protein was essential for tctI function, and it was found to be weakly attached to the inner membrane. Thus tctI encodes four proteins, one periplasmic, two integral, and one peripheral to the cytoplasmic membrane, with the genes arranged as tctA tctB tctC tctD.     

Alterations in tubulin immunoreactivity; relation to secondary structure

Blood sinusoidal plasma membrane subfractions were isolated from normal mouse liver in the presence of the proteinase inhibitors PhMeSO2F and iodoacetamide. They were purified from smooth microsomal and Golgi vesicle contaminants. The phosphorylation reaction was studied at 33 degrees C, in the presence of 2 mM MnCl2. Addition of epidermal growth factor (EGF) to the preparations stimulated 32P incorporation from [gamma-32P]ATP or [gamma-32P]GTP essentially into one 170 000 Mr protein. Some incorporation was observed in a minor 120 000-Mr component which appears to be a degradation product of the 170 000-Mr component. No EGF-dependent phosphorylation of other membrane proteins or various exogenous proteins could be detected in vitro. The dephosphorylation of the 170 000-Mr component was observed after 4 min of incubation at 33 degrees C. This dephosphorylation reaction was inhibited by addition of 5 mM p-nitrophenyl phosphate but not by addition of micromolar Zn2+, Be2+ or orthovanadate. The 170 000-Mr protein specifically bound 125I-labeled EGF and thus appeared to be the hepatic EGF receptor. The EGF stimulatable kinase activity considerably enhances incorporation of 32P into tyrosine residues of the 170 000-Mr EGF receptor at 33 degrees C. Tryptic peptide maps of the 32P-labeled 170 000-Mr protein revealed a multiplicity of phosphorylated sites. Seven 32P-labeled phosphopeptides were observed after EGF stimulation, three of them being largely prominent. Tryptic peptide maps of the 170 000-Mr protein after it was covalently linked to 125I-labeled EGF showed only one 125I-labeled peptide, the migration of which appeared different from that of 32P-labeled phosphopeptides. These findings were confirmed by V8 protease unidimensional peptide mapping of the 170 000-Mr protein, labeled with 32P or 125I-EGF.     

Characterization of Porins Isolated from the Outer Membrane of Serratia liquefaciens

A major outer membrane protein with an apparent molecular weight of 42 kDa was purified from Serratia liquefaciens grown on Brain Heart Infusion medium. The same protein was obtained when the cells were grown on a synthetic medium supplemented with 2% glucose. The amino acid composition of this protein revealed it to be hydrophilic. The pore-forming ability of the 42-kDa protein was determined by the liposome swelling assay. This assay demonstrated that the protein forms nonspecific channels with a diameter between 1.16 and 1.6 nm. An additional protein with a molecular weight of 47 kDa was obtained on synthetic medium supplemented with maltose. This protein exhibited specific pore-forming ability to maltose and maltodextrins, but was also permeable to other compounds, according to their size. When bacteria were grown on Nutrient Broth medium, two outer membrane proteins with molecular weights of 41 kDa and 42 kDa were produced by the bacteria. All three types of proteins represent monomers of respective oligomers. The monomers did not exhibit pore-forming ability when incorporated into liposomes. We, therefore, propose that the oligomer is the functional unit of a porin capable of forming permeability channels in the outer membrane of Serratia liquefaciens. These results indicate that S. liquefaciens contains several porins exhibiting specific osmoregulation or that are induced by a specific nutrient, where the 42-kDa outer membrane protein of this bacterium is certainly a major porin. Received: 6 July 1998 / Accepted: 19 August 1998     

Phosphorylation in vitro of membrane fragments from Torpedo marmorata electric organ. Effect on membrane solubilization by detergents

Acetylcholine receptor-rich membrane fragments purified from Torpedo marmorata electric organ were phosphorylated, in vitro, by endogenous protein kinases. The 40 000-Mr chain, which carries the acetylcholine receptor site, was never labelled; on the other hand, protein bands of apparent molecular weights 43 000, 50 000 and 66 000, which are present in the acetylcholine receptor-rich membranes, were repeatedly phosphorylated. The phosphorylation of these three peptides required the presence of divalent cations, such as Mg2+ or Mn2+, and was, in addition, stimulated up to 3--5-fold by K+. The effect of Na+ ions appeared less specific since Na+ ions reduced the labelling of all the polypeptides susceptible to phosphorylation. Cholinergic agonists and antagonists, local anesthetics and cyclic nucleotides did not affect the phosphorylation of the receptor-rich membranes. Phosphorylation selectively modified the solubilization of several polypeptides by nondenaturing detergents: phosphorylated 43 000-Mr, 50 000-Mr and 66 000-Mr polypeptides were solubilized at lower concentrations of detergent than their non-phosphorylated counterparts. Two-dimensional gels revealed the existence of a charge heterogeneity of the 40 000-Mr and 43 000-Mr chains. The microheterogeneity of the 43 000-Mr chain, but not that of the 40 000-Mr chain, might result from a selective phosphorylation of this particular chain.     

Isolation of streptococcal hyaluronate synthase.

Hyaluronate synthase was isolated from protoblast membranes of streptococci by Triton X-114 extraction and cetylpyridinium chloride precipitation. It was identified as a 52,000-Mr protein, which bound to nascent hyaluronate and was affinity-labelled by periodate-oxidized UDP-glucuronic acid and UDP-N-acetylglucosamine. Antibodies directed against the 52,000-Mr protein inhibited hyaluronate synthesis. Mutants defective in hyaluronate synthase activity lacked the 52,000-Mr protein in membrane extracts. Synthase activity was solubilized from membranes by cholate in active form and purified by ion-exchange chromatography.     

Regulated Secretion of Pro-Opiomelanocortin Converting Enzyme and an Aminopeptidase B-Like Enzyme from Dispersed Bovine Intermediate Lobe Pituitary Cells

Coordinate secretion of two prohormone/proneuropeptide processing enzymes [pro-opiomelanocortin converting enzyme (PCE) and an aminopeptidase B-like enzyme (APBE)] and alpha-melanotropin (alpha-MSH) from bovine intermediate lobe pituitary cells was studied. Stimulation of secretion with 8-bromo-cyclic AMP produced significant increases in levels of immunoreactive alpha-MSH, PCE, and APBE. Treatment of cells with the dopaminergic agonist 2-bromo-alpha-ergocryptine resulted in significant decreases in secretion of alpha-MSH, PCE, and APBE. In neither case were there significant changes in levels of cytosolic lactic dehydrogenase or lysosomal beta-glucuronidase in the medium. The secreted PCE activity was shown to process frog and mouse pro-opiomelanocortin primarily to 23,000-Mr corticotropin (ACTH), 13,000-Mr ACTH, beta-lipotropin, a beta-endorphin-like peptide, and beta-endorphin, products comparable to those synthesized by the mouse and frog intermediate lobe in situ. The secreted enzymatic activity had a pH optimum between 4.0 and 5.0, was strongly inhibited by pepstatin A, and had an inhibitor profile similar to the purified bovine intermediate lobe PCE. The secreted APBE activity cleaved Argo-[Met]-enkephalin to [Met]-enkephalin and had a pH optimum and inhibitor profile similar to that previously reported for an activity from purified secretory vesicle fractions of bovine intermediate and neural lobes. The coordinate regulated secretion of alpha-MSH and enzyme activities (PCE and APBE) strongly indicates their colocalization in the same secretory vesicle compartment within the cell. The characteristics of the two enzymes secreted in the medium paralleled those seen in the tissue and further support their role in pro-opiomelanocortin processing in vivo.     

Iron- and molybdenum-repressible outer membrane proteins in competent Azotobacter vinelandii.

Azotobacter vinelandii produced three major proteins of 93,000, 85,000, and 81,000 daltons and a minor 77,000-dalton protein in the outer membrane of Fe-limited cells, and these cells were competent for transformation by DNA. The synthesis of these proteins was repressed in Fe-sufficient medium. Mo limitation of nitrogen-fixing cells resulted in the hyperproduction of a 44,000-dalton protein and the production of a minor 77,000-dalton protein in the outer membrane. Mo limitation enhanced competence in Fe-limited medium and induced competence in Fe-sufficient medium. The 44,000-dalton protein was replaced by a 45,000-dalton protein when Fe-sufficient medium also contained NH4+, but the cells were noncompetent. The synthesis of these proteins was repressed in Mo-sufficient medium and by NH4+ in Fe-limited medium. All of the culture supernatants contained a blue-white fluorescent material (absorbance maximum, 214 nm) which appeared to coordinate Fe3+, Fe2+, MoO4(2-), WO3(2-), and VO3(-).     

Hydroxamate production by Aquaspirillum magnetotacticum.

Spent culture fluids from Aquaspirillum magnetotacticum MS-1 grown at high (20 microM) but not low (5 microM) iron concentration contained material yielding a positive hydroxamate test. Cells possessed six major outer membrane proteins. Three outer membrane proteins ranging from 72,000 to 85,000 daltons were coordinately produced at iron concentrations conducive to hydroxamate production. A 55,000-dalton iron-repressible outer membrane protein was also present in strain MS-1 cultured at low but not high ferric quinate concentration. Culture fluids from strain MS-1 which were hydroxamate positive augmented growth of a Salmonella typhimurium siderophore-deficient (enb-7) mutant in low-iron medium, suggesting a role of hydroxamate in uptake of iron by the cell.     

Involvement of protein kinase C in the phosphorylation of an insulin-granule membrane protein.

The involvement of protein kinase C in the Ca2+-dependent phosphorylation of a 29 000-Mr insulin-granule membrane protein prepared from a rat insulinoma was investigated. Protein kinase C activity towards exogenous lysine-rich histone was detected in a cytosolic fraction prepared from an insulinoma homogenate in the presence of EGTA. This activity bound reversibly to insulin granules in a Ca2+-dependent manner. Phosphatidylserine liposomes removed both protein kinase C activity and the 29 000-Mr protein-phosphorylating activity from the cytosolic fraction in a Ca2+-dependent fashion. Protein kinase C activity and the enzymic activity responsible for the phosphorylation of the 29 000-Mr granule protein behaved identically on sucrose-density-gradient centrifugation, ion-exchange chromatography, (NH4)2SO4 fractionation and gel filtration of the cytosolic fraction. These results are consistent with protein kinase C being the enzyme responsible for the phosphorylation of the 29 000-Mr insulin-granule membrane protein.     

Chloroplast phosphoproteins. Evidence for a thylakoid-bound phosphoprotein phosphatase

1. Isolated intact pea (Pisum sativum) chloroplasts incorporate [32P]orthophosphate into several thylakoid polypeptides in the light. Transfer of the labelled chloroplasts to darkness results in rapid dephosphorylation of the polypeptides. The most rapidly dephosphorylated phosphoproteins are the 26000-Mr doublet derived from the light-harvesting chlorophyll a/b binding complex. 2. Incubation of isolated 32P-labelled thylakoids in buffer in the absence of stromal components also results in rapid protein dephosphorylation. Again, the most rapidly dephosphorylated phosphoproteins are the 26000-Mr light-harvesting doublet. Dephosphorylation of all thylakoid phosphoproteins is accelerated by addition of up to 10 mM MgCl2. 3. The enzyme responsible for dephosphorylation is a phosphatase rather than a phosphotransferase or the thylakoid protein kinase acting in reverse. The enzyme is specifically and totally inhibited by NaF and does not require phosphoryl group acceptors such as ADP. Unlike the protein kinase, the phosphatase is indifferent to light and the electron transport inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea. 4. The phosphorylated regions of the thylakoid phosphoproteins protrude from the outer surface of the membrane and are removed by trypsin treatment.     

Resistance of Pseudomonas aeruginosa to isothiazolone

This investigation was to determine whether Pseudomonas aeruginosa could acquire resistance to the bactericide isothiazolone, and what the nature of such a resistance mechanism would be. The Pseudomonas was cultured in nutrient-limited broth in the presence of sub-inhibitory concentrations of isothiazolone (a mixture of 1.15% 5-chloro- N -methylisothiazolone (CMIT) and 0.35% N -methylisothiazolone (MIT)). Three cultures tested in parallel adapted gradually during exposure for 15 d from an initial minimum inhibitory concentration (MIC) of 300 μl 1^-1 to 607 μl ^-1. The three parallel cultures adapted at similar rates, so the adaptation was not ascribed to mutation but to a specific mechanism. Resistant cells did not produce any extracellular isothiazolone-quenching compounds nor undergo detectable alterations in their lipopolysaccharide layer. In wild cells, a 35 kDa outer membrane protein (protein T) was detectable, whereas resistant cells lacked this protein. Production of protein T was suppressed within 24 h of exposure to isothiazolone. It was still suppressed after 72 h of growth in isothiazolone-free medium. It is proposed that Ps. aeruginosa acquires resistance to isothiazolone by a process of adaptation where the outer membrane protein T is suppressed.     

Purification and characterization of the phosphorylated DNA-binding protein from adenovirus-type-2-infected cells.

The virus-coded 72000-Mr DNA-binding protein from adenovirus-type-2-infected cells has been purified to homogeneity by DEAE-cellulose chromatography, selective precipitation and gel filtration. The 72000-Mr DNA-binding protein is phosphorylated and the phosphate is covalently linked predominantly to serine. Analysis of tryptic digests of the 32P-labeled 72000-Mr protein showed that the phosphate residue(s) is present in only one peptide. The DNA-binding fraction contains an additional non-phosphorylated protein with an approximate molecular weight of 45000. Tryptic peptide maps of [35S]methionine-labeled 72000-Mr and 45000-Mr polypeptides are indistinguishable. The amino acid compositions of the 72000-Mr and 45000-Mr polypeptides show closely related distributions. An antiserum produced against the purified 72000-Mr DNA-binding protein precipitates both the 72000-Mr and the 45000-Mr protein from extracts of adenovirus-infected cells. Immunofluorescence studies revealed DNA-binding protein to be accumulated in characteristic structures in nuclei of the infected cells.     

Identification of a lipid-anchored heparan sulfate proteoglycan in Schwann cells

Schwann cells synthesize both hydrophobic and peripheral cell surface heparan sulfate proteoglycans (HSPGs). Previous analysis of the kinetics of radiolabeling suggested the peripheral HSPGs are derived from the membrane-anchored forms (Carey, D., and D. Evans. 1989. J. Cell Biol. 108:1891-1897). Peripheral cell surface HSPGs were purified from phytic acid extracts of cultured neonatal rat sciatic nerve Schwann cells by anion exchange, gel filtration, and laminin-affinity chromatography. Approximately 250 micrograms of HSPG protein was obtained from 2 X 10(9) cells with an estimated recovery of 23% and an overall purification of approximately 2000-fold. SDS-PAGE analysis indicated the absence of non-HSPG proteins in the purified material. Analysis of heparinase digestion products revealed the presence of at least six core protein species ranging in molecular weight from 57,000 to 185,000. The purified HSPGs were used to produce polyclonal antisera in rabbits. The antisera immunoprecipitated a subpopulation of 35SO4- labeled HSPGs that were released from Schwann cells by incubation in medium containing phosphatidylinositol-specific phospholipase C (PI- PLC); smaller amounts of immunoprecipated HSPGs were also present in phytic acid extracts. In the presence of excess unlabeled PI-PLC- released proteins, immunoprecipitation of phytic acid-solubilized HSPGs was inhibited. SDS-PAGE analysis of proteins immunoprecipitated from extracts of [35S]methionine labeled Schwann cells demonstrated that the antisera precipitated an HSPG species that was present in the pool of proteins released by PI-PLC, with smaller amounts present in phytic acid extracts. Nitrous acid degradation of the immunoprecipitated proteins produced a single 67,000-Mr core protein. When used for indirect immunofluorescence labeling, the antisera stained the external surface of cultured Schwann cells. Preincubation of the cultures in medium containing PI-PLC but not phytic acid significantly reduced the cell surface staining. The antisera stained the outer ring of Schwann cell membrane in sections of adult rat sciatic nerve but did not stain myelin or axonal membranes. This localization suggests the HSPG may play a role in binding the Schwann cell plasma membrane to the adjacent basement membrane surrounding the individual axon-Schwann cell units.     

Lipid fluidity-dependent biosynthesis and assembly of the outer membrane proteins of E. coli.

The reduction of the membrane lipids of E. coli to a nonfluid state resulted in the accumulation in the cell envelope of a high molecular weight precursor of the protoIG protein, a major outer membrane protein. The protoIG protein was as sensitive to trypsin as the mature toIG protein assembled in the outer membrane. In contrast to the toIG protein, however, the accumulated protoIG protein was easily released from the envelope fraction by both sodium lauryl sarcosinate extraction and sonication. This indicated that the precursor protein was loosely associated with the cell membrane. When a fluid lipid state was restored, the protoIG protein was processed to the mature form which was then correctly assembled in the outer membrane. These results suggest that the protoIG protein produced under nonfluid lipid conditions was properly translocated across the cytoplasmic membrane, but could not be assembled in the outer membrane due either to the reversible inhibition of the processing of the ProtoIG to the toIG protein or to the lack of interaction with a specific outer membrane component(s). Reduced lipid fluidity also caused various alterations in the biosynthesis and assembly of other membrane proteins. In addition to the toIG protein, a large number of new proteins were accumulated in the membrane. Alternatively, the matrix protein as well as the promatrix protein were not detected in the cell envelope. On the other hand, the lipoprotein was normally produced, processed, modified and assembled in the outer membrane. These results indicate that the outer membrane proteins are synthesized and assembled according to several different mechanisms, on which the physical state of the membrane has various effects.     

Bacterial outer membrane secretin PulD assembles and inserts into the inner membrane in the absence of its pilotin

Dodecamerization and insertion of the outer membrane secretin PulD is entirely determined by the C-terminal half of the polypeptide (PulD-CS). In the absence of its cognate chaperone PulS, PulD-CS and PulD mislocalize to the inner membrane, from which they are extractable with detergents but not urea. Electron microscopy of PulD-CS purified from the inner membrane revealed apparently normal dodecameric complexes. Electron microscopy of PulD-CS and PulD in inner membrane vesicles revealed inserted secretin complexes. Mislocalization of PulD or PulD-CS to this membrane induces the phage shock response, probably as a result of a decreased membrane electrochemical potential. Production of PulD in the absence of the phage shock response protein PspA and PulS caused a substantial drop in membrane potential and was lethal. Thus, PulD-CS and PulD assemble in the inner membrane if they do not associate with PulS. We propose that PulS prevents premature multimerization of PulD and accompanies it through the periplasm to the outer membrane. PulD is the first bacterial outer membrane protein with demonstrated ability to insert efficiently into the inner membrane.     

G-proteins of fat-cells. Role in hormonal regulation of intracellular inositol 1,4,5-trisphosphate.

Pertussis toxin abolishes hormonal inhibition of adenylate cyclase, hormonal stimulation of inositol 1,4,5-trisphosphate accumulation in rat fat-cells, and catalyses the ADP-ribosylation of two peptides, of Mr 39,000 and 41,000 [Malbon, Rapiejko & Mangano (1985) J. Biol. Chem. 260, 2558-2564]. The 41,000-Mr peptide is the alpha-subunit of the G-protein, referred to as Gi, that is believed to mediate inhibitory control of adenylate cyclase by hormones. The nature of the 39,000-Mr substrate for pertussis toxin was investigated. The fat-cell 39,000-Mr peptide was compared structurally and immunologically with the alpha-subunits of two other G-proteins, Gt isolated from the rod outer segments of bovine retina and Go isolated from bovine brain. After radiolabelling in the presence of pertussis toxin and [32P]NAD+, the electrophoretic mobilities of the fat-cell 39,000-Mr peptide and the alpha-subunits of Go and Gt were nearly identical. Partial proteolysis of these ADP-ribosylated proteins generates peptide patterns that suggest the existence of a high degree of homology between the fat-cell 39,000-Mr peptide and the alpha-subunit of Go. Antisera raised against purified G-proteins and their subunits were used to probe immunoblots of purified Gt, Gi, Go, and fat-cell membrane proteins. Although recognizing the 36,000-Mr beta-subunit band of Gt, Gi, Go and a 36,000-Mr fat-cell peptide, antisera raised against Gt failed to recognize either the 39,000- or the 41,000-Mr peptides of fat-cells or the alpha-subunits of Go and Gi. Antisera raised against the alpha-subunit of Go, in contrast, recognized the 39,000-Mr peptide of rat fat-cells, but not the alpha-subunit of either Gi or Gt. These data establish the identity of Go, in addition to Gi, in fat-cell membranes and suggest the possibility that either Go or Gi alone, or both, may mediate hormonal regulation of adenylate cyclase and phospholipase C.