Organization of the genes encoding [Fe] hydrogenase in Desulfovibrio vulgaris subsp. oxamicus Monticello.

The genes encoding the periplasmic [Fe] hydrogenase from Desulfovibrio vulgaris subsp. oxamicus Monticello were cloned by exploiting their homology with the hydAB genes from D. vulgaris subsp. vulgaris Hildenborough, in which this enzyme is present as a heterologous dimer of alpha and beta subunits. Nucleotide sequencing showed that the enzyme is encoded by an operon in which the gene for the 46-kilodalton (kDa) alpha subunit precedes that of the 13.5-kDa beta subunit, exactly as in the Hildenborough strain. The pairs of hydA and hydB genes are highly homologous; both alpha subunits (420 amino acid residues) share 79% sequence identity, while the unprocessed beta subunits (124 and 123 amino acid residues, respectively) share 71% sequence identity. In contrast, there appears to be no sequence homology outside these coding regions, with the exception of a possible promoter element, which was found approximately 90 base pairs upstream from the translational start of the hydA gene. The recently discovered hydC gene, which may code for a 65.8-kDa fusion protein (gamma) of the alpha and beta subunits and is present immediately downstream from the hydAB genes in the Hildenborough strain, was found to be absent from the Monticello strain. The implication of this result for the possible function of the hydC gene product in Desulfovibrio species is discussed.     

Substrate-specific selenoprotein B of glycine reductase from Eubacterium acidaminophilum. Biochemical and molecular analysis.

The substrate-specific selenoprotein B of glycine reductase (PBglycine) from Eubacterium acidaminophilum was purified and characterized. The enzyme consisted of three different subunits with molecular masses of about 22 (alpha), 25 (beta) and 47 kDa (gamma), probably in an alpha 2 beta 2 gamma 2 composition. PBglycine purified from cells grown in the presence of [75Se]selenite was labeled in the 47-kDa subunit. The 22-kDa and 47-kDa subunits both reacted with fluorescein thiosemicarbazide, indicating the presence of a carbonyl compound. This carbonyl residue prevented N-terminal sequencing of the 22-kDa (alpha) subunit, but it could be removed for Edman degradation by incubation with o-phenylenediamine. A DNA fragment was isolated and sequenced which encoded beta and alpha subunits of PBglycine (grdE), followed by a gene encoding selenoprotein A (grdA2) and the gamma subunit of PBglycine (grdB2). The cloned DNA fragment represented a second GrdB-encoding gene slightly different from a previously identified partial grdBl-containing fragment. Both grdB genes contained an in-frame UGA codon which confirmed the observed selenium content of the 47-kDa (gamma) subunit. Peptide sequence analyses suggest that grdE encodes a proprotein which is cleaved into the previously sequenced N-terminal 25-kDa (beta) subunit and a 22-kDa (alpha) subunit of PBglycine. Cleavage most probably occurred at an -Asn-Cys- site concomitantly with the generation of the blocking carbonyl moiety from cysteine at the alpha subunit.     

Major pertussis-toxin-sensitive GTP-binding protein of bovine lung. Purification, characterization and production of specific antibodies

Two GTP-binding proteins which can be ADP-ribosylated by islet-activating protein, pertussis toxin, were purified from the cholate extract of bovine lung membranes. Both proteins had the same heterotrimeric structure (alpha beta gamma), but the alpha subunits were dissociated from the beta gamma when they were purified in the presence of AlCl3, MgCl2 and NaF. The molecular mass of the alpha subunit of the major protein (designated GLu, with beta gamma) was 40 kDa and that of the minor one was 41 kDa. The results of peptide mapping analysis of alpha subunits with a limited proteolysis indicated that GLu alpha was entirely different from the alpha of brain Gi or Go, while the 41-kDa polypeptide was identical with the alpha of bovine brain Gi. The kinetics of guanosine 5'-[3-O-thio]triphosphate (GTP[gamma S]) binding to GLu was similar to that to lung Gi but quite different from that to brain Go. On the other hand, incubation of GLu alpha at 30 degrees C caused a rapid decrease of GTP[gamma S] binding, the inactivation curve being similar to that of Go alpha but different from that of Gi alpha. The alpha subunits of lung Gi and GLu did not react with the antibodies against the alpha subunit of bovine brain Go. The antibodies were raised in rabbits against GLu alpha and were purified with a GLu alpha-Sepharose column. The purified antibodies reacted not only with GLu alpha but also with the 41-kDa protein and purified brain Gi alpha. However, the antibodies adsorbed with brain Gi alpha reacted only with GLu alpha, indicating antisera raised with GLu alpha contained antibodies that recognize both Gi alpha and GLu alpha, and those specific to GLu alpha. These results further indicate that GLu is different from Gi or Go. Anti-GLu alpha antibodies reacted with the 40-kDa proteins in the membranes of bovine brain and human leukemic (HL-60) cells. The beta gamma subunits were also purified from bovine lung. The beta subunit was the doublet of 36-kDa and 35-kDa polypeptides. The lung beta gamma could elicit the ADP-ribosylation of GLu alpha by islet-activating protein, increase the GTP[gamma S] binding to GLu and protect the thermal denaturation of GLu alpha. The antibodies raised against brain beta gamma cross-reacted with lung beta but not with lung gamma.     

A chimera-like alpha-amylase inhibitor suggesting the evolution of Phaseolus vulgaris alpha-amylase inhibitor

White kidney bean (Phaseolus vulgaris) contains two kinds of alpha-amylase inhibitors, one heat-stable (alpha AI-s) and one heat-labile (alpha AI-u). alpha AI-s has recently been revealed to be a tetrameric complex, alpha(2)beta(2), with two active sites [Kasahara et al. (1996) J. Biochem. 120, 177-183]. The present study was undertaken to reveal the molecular features of alpha AI-u, which is composed of three kinds of subunits, alpha, beta, and gamma. The gamma-subunit, in contrast to the alpha- and beta-subunits that are indistinguishable from the alpha- and beta-subunits of alpha AI-s, was found to correspond to a subunit of an alpha-amylase inhibitor-like protein, which has been identified as an inactive, evolutionary intermediate between arcelin and the alpha-amylase inhibitor in a P. vulgaris defense protein family. The polypeptide molecular weight of alpha AI-u determined by the light-scattering technique, together with the polypeptide molecular weights of the subunits, suggests that alpha AI-u is a trimeric complex, alpha beta gamma. The inhibition of alpha AI-u by increasing amounts of porcine pancreatic alpha-amylase (PPA) indicates that an inactive 1:1 complex is formed between alpha AI-u and PPA. Molecular weight estimation of the complex by the light-scattering technique confirmed that it is a complex of alpha AI-u with one PPA molecule. Thus it seems probable that alpha AI-u is an evolutionary intermediate of the P. vulgaris alpha-amylase inhibitor.     

Characterization of heterotrimeric G protein complexes in rice plasma membrane

Two genes in the rice genome were identified as those encoding the gamma subunits, gamma1 and gamma2, of heterotrimeric G proteins. Using antibodies against the recombinant proteins for the alpha, beta, gamma1, and gamma2 subunits of the G protein complexes, all of the subunits were proven to be localized in the plasma membrane in rice. Gel filtration of solubilized plasma membrane proteins showed that all of the alpha subunits were present in large protein complexes (about 400 kDa) containing the other subunits, beta, gamma1, and gamma2, and probably also some other proteins, whereas large amounts of the beta and gamma (gamma1 and gamma2) subunits were freed from the large complexes and took a 60-kDa form. A yeast two-hybrid assay and co-immunoprecipitation experiments showed that the beta subunit interacted tightly with the gamma1 and gamma2 subunits, and so the beta and gamma subunits appeared to form dimers in rice cells. Some dimers were associated with the alpha subunit, because few beta, gamma1, and gamma2 subunits were present in the 400-kDa complexes in a rice mutant, d1, which was lacking in the alpha subunit. When a constitutively active form of the alpha subunit was prepared by the exchange of one amino acid residue and introduced into d1, the mutagenized subunit was localized in the plasma membrane of the transformants and took a free, and not the 400-kDa, form.     

hydγ, a gene from Desulfovibrio vulgaris (Hildenborough) encodes a polypeptide homologous to the periplasmic hydrogenase

Downstream of the genes for the structural alpha and beta subunits of the periplasmic Desulfovibrio vulgaris (Hildenborough) hydrogenase a DNA fragment was detected with sequence homology to these genes. This fragment was cloned in Escherichia coli and the nucleotide sequence was determined. A gene was detected on the fragment with coding capacity for a 65.8 kDa polypeptide, hyd gamma. The central part of hyd gamma has an unusually high degree of homology with the alpha subunit and the C-terminal part has similarity with the beta subunit. These results strongly suggest that the three genes for hyd gamma and the alpha and beta subunits derive from one common ancestor gene. We succeeded in the identification of the translational product of this gene in E. coli, but were unable to determine the function of hyd gamma after expression in E. coli.     

The NH2-terminal alpha subunit attenuator domain confers regulation of G protein activation by beta gamma complexes.

Gs and Gi, respectively, activate and inhibit the enzyme adenylyl cyclase. Regulation of adenylyl cyclase by the heterotrimeric Gs and Gi proteins requires the dissociation of GDP and binding of GTP to the alpha s or alpha i subunit. The beta gamma subunit complex of Gs and Gi functions, in part, to inhibit GDP dissociation and alpha subunit activation by GTP. Multiple beta and gamma polypeptides are expressed in different cell types, but the functional significance for this heterogeneity is unclear. The beta gamma complex from retinal rod outer segments (beta gamma t) has been shown to discriminate between alpha i and alpha s subunits (Helman et al: Eur J Biochem 169:431-439, 1987). beta gamma t efficiently interacts with alpha i-like G protein subunits, but poorly recognizes the alpha s subunit. beta gamma t was, therefore, used to define regions of the alpha i subunit polypeptide that conferred selective regulation compared to the alpha s polypeptide. A series of alpha subunit chimeras having NH2-terminal alpha i and COOH-terminal alpha s sequences were characterized for their regulation by beta gamma t, measured by the kinetics of GTP gamma S activation of adenylyl cyclase. A 122 amino acid NH2-terminal region of the alpha i polypeptide encoded within an alpha i/alpha s chimera was sufficient for beta gamma t to discriminate the chimera from alpha s. A shorter 54 amino acid alpha i sequence substituted for the corresponding NH2-terminal region of alpha s was insufficient to support the alpha i-like interaction with beta gamma t. The findings are consistent with our previous observation (Osawa et al: Cell 63:697-706, 1990) that a region in the NH2-terminal moiety functions as an attenuator domain controlling GDP dissociation and GTP activation of the alpha subunit polypeptide and that the attenuator domain is involved in functional recognition and regulation by beta gamma complexes.     

Characterization of the 1,4-dihydropyridine receptor using subunit-specific polyclonal antibodies. Evidence for a 32,000-Da subunit

The purified receptor for the 1,4-dihydropyridine Ca2+ channel blockers from rabbit skeletal muscle contains protein components of 170,000 Da (alpha 1), 175,000 Da (alpha 2), 52,000 Da (beta), and 32,000 Da (gamma) when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. Subunit-specific polyclonal antibodies have now been prepared and used to characterize the association of the 32,000-Da polypeptide (gamma subunit) with other subunits of the dihydropyridine receptor. Immunoblot analysis of fractions collected during purification of the dihydropyridine receptor shows that the 32,000-Da polypeptide copurified with alpha 1 and alpha 2 subunits at each step of the purification. In addition, monoclonal antibodies against the alpha 1 and beta subunits immunoprecipitate the digitonin-solubilized dihydropyridine receptor as a multisubunit complex which includes the 32,000-Da polypeptide. Polyclonal antibodies generated against both the nonreduced and reduced forms of the alpha 2 subunit and the gamma subunit have been used to show that the 32,000-Da polypeptide is not a proteolytic fragment of a larger component of the dihydropyridine receptor and not disulfide linked to the alpha 2 subunit. In addition, polyclonal antibodies against the rabbit skeletal muscle 32,000-Da polypeptide specifically react with similar proteins in skeletal muscle of other species including avian and amphibian species. Thus, our results demonstrate that the 32,000-Da polypeptide (gamma subunit) is an integral and distinct component of the dihydropyridine receptor.     

Antisense suppression of GABA(A) receptor beta subunit levels in cultured cerebellar granule neurons demonstrates their importance in receptor expression

GABA(A) receptors in the CNS are pentameric molecules composed of alpha, beta, gamma, delta, epsilon and theta subunits. Studies on transfected cells have shown that GABA(A) receptor beta subunit isoforms can direct alpha1 subunit localization within the cell. To examine the role of selected subunits in governing GABA(A) receptor expression in neurons, cultures of rat cerebellar granule cells were grown with antisense or sense oligodeoxynucleotides (ODNs) specific for the alpha 1, beta 2 or gamma 2 subunits. These subunits are all expressed in granule neurons where they are thought to contribute to an abundant receptor type. Following ODN treatment, subunit expression and distribution were examined by western blotting, immunocytochemistry and RT-PCR. Treatment of the cultures with the antisense, but not the corresponding sense, ODNs reduced the levels of the targeted subunit polypeptides. In addition, the beta 2 antisense ODN reduced the level of the alpha1 subunit polypeptide without altering the level of its mRNA. In contrast, treatment with the beta 2 subunit antisense ODN did not alter gamma 2 subunit polypeptide expression, distribution or mRNA level. These findings suggest that the alpha1 subunit requires a beta subunit for assembly into GABA(A) receptors in cerebellar granule neurons.     

Endoplasmic reticulum retention and associated degradation of a GABAA receptor epilepsy mutation that inserts an aspartate in the M3 transmembrane segment of the alpha1 subunit

A GABA(A) receptor alpha1 subunit epilepsy mutation (alpha1(A322D)) introduces a negatively charged aspartate residue into the hydrophobic M3 transmembrane domain of the alpha1 subunit. We reported previously that heterologous expression of alpha1(A322D)beta2gamma2 receptors in mammalian cells resulted in reduced total and surface alpha1 subunit protein. Here we demonstrate the mechanism of this reduction. Total alpha1(A322D) subunit protein was reduced relative to wild type protein by a similar amount when expressed alone (86 +/- 6%) or when coexpressed with beta2 and gamma2S subunits (78 +/- 6%), indicating an expression reduction prior to subunit oligomerization. In alpha1beta2gamma2S receptors, endoglycosidase H deglycosylated only 26 +/- 5% of alpha1 subunits, consistent with substantial protein maturation, but in alpha1(A322D)beta2gamma2S receptors, endoglycosidase H deglycosylated 91 +/- 4% of alpha1(A322D) subunits, consistent with failure of protein maturation. To determine the cellular localization of wild type and mutant subunits, the alpha1 subunit was tagged with yellow (alpha1-YFP) or cyan (alpha1-CFP) fluorescent protein. Confocal microscopic imaging demonstrated that 36 +/- 4% of alpha1-YFPbeta2gamma2 but only 5 +/- 1% alpha1(A322D)-YFPbeta2gamma2 colocalized with the plasma membrane, whereas the majority of the remaining receptors colocalized with the endoplasmic reticulum (55 +/- 4% alpha1-YFPbeta2gamma2S, 86 +/- 3% alpha1(A322D)-YFP). Heterozygous expression of alpha1-CFPbeta2gamma2S and alpha1(A322D)-YFPbeta2gamma2S or alpha1-YFPbeta2gamma2S and alpha1(A322D)-CFPbeta2gamma2S receptors showed that membrane GABA(A) receptors contained primarily wild type alpha1 subunits. These data demonstrate that the A322D mutation reduces alpha1 subunit expression after translation, but before assembly, resulting in endoplasmic reticulum-associated degradation and membrane alpha1 subunits that are almost exclusively wild type subunits.     

Activation and solubilization of the retinal cGMP-specific phosphodiesterase by limited proteolysis. Role of the C-terminal domain of the beta-subunit.

The cGMP-specific phosphodiesterase (PDE) of vertebrate retinal rod outer segments (ROS) is a peripheral enzyme activated in vivo by transducin. In vitro artificial activation can be achieved using trypsin. This was described as resulting from degradation of the inhibitory gamma subunit (2 copies/PDE molecule), leaving intact the alpha beta catalytic core. It was, however, observed that trypsin could induce the release of PDE (or solubilization) from the ROS membranes before its activation [Wensel, T. G. & Stryer, L. (1986) Proteins Struct. Funct. Genet. 1, 90-99]. Studying the time course of this solubilization, we were able to purify a trypsin-solubilized PDE still completely inhibited (i.e. with its two gamma subunits bound). The tryptic solubilization of PDE is therefore complete before any functional degradation of the gamma subunits occurs. It was recently suggested that this solubilization could coincide with the cleavage of a C-terminal fragment of the alpha subunit, which can be labeled by methylation of a terminal cysteine residue [Ong, O. C., Ota, I. M., Clarke, S. & Fung, B. K. K. (1989) Proc. Natl Acad. Sci. USA 86, 9238-9242]. We present the following evidence indicating that the C-terminus of the PDE beta subunit is mainly responsible for PDE anchorage to the ROS membrane. (a) The trypsin-solubilized PDE alpha beta gamma 2 has intact blocked N-termini. (b) It is still methylated on PDE alpha. (c) The C-terminus of PDE beta can also be labeled by methylation and its tryptic cleavage coincides well with the PDE solubilization. (d) Sequential cleavage of the alpha and beta polypeptides can also be detected by high-resolution gel electrophoresis: the first cleavage appears on the beta subunit and is completed when cleavage of the alpha subunit begins. The time course for cleavage of the gamma subunits appears to be slower than for the beta subunit and comparable to that of the alpha subunit. Upon longer trypsinization, a 70-kDa polypeptide appears which seems to be a degradation product of PDE beta. Gel-filtration analysis, however, shows that this 70-kDa fragment does not dissociate from the catalytic core.     

Role of the subunits of the energy-transducing adenosine triphosphatase from Micrococcus lysodeikticus membranes studied by proteolytic digestion and immunological approaches.

An energy-transducing adenosine triphosphatase (ATPase, EC 3.6.1.3) that contains an extra polypeptide (delta) as well as three intrinsic subunits (alpha, beta, gamma) was purified from Micrococcus lysodeikticus membranes. The apparent subunit stoichiometry of this soluble ATPase complex is alpha 3 beta 3 gamma delta. The functional role of the subunits was studied by correlating subunit sensitivity to trypsin and effect of antibodies raised against holo-ATPase and its alpha, beta and gamma subunits with changes in ATPase activity and ATPase rebinding to membranes. A form of the ATPase with the subunit proportions 1.67(alpha):3.00(beta:0.17(gamma) was isolated after trypsin treatment of purified ATPase. This form has more than twice the specific activity of native enzyme. Other forms with less relative proportion of alpha subunits and absence of gamma subunit are not active. Of the antisera to subunits, only anti-(beta-subunit) serum shows a slight inhibitory effect on ATPase activity, but its combination with either anti-(alpha-subunit) or anti-(gamma-subunit) serum increases the effect. The results suggest that beta subunit is required for full ATPase activity, although a minor proportion of alpha and perhaps gamma subunit(s) is also required, probably to impart an active conformation to the protein. The additional polypeptide not hitherto described in Micrococcus lysodeikticus ATPase had a molecular weight of 20 000 and was found to be involved in ATPase binding to membranes. This 20 000-dalton component can be equated with the delta subunit of other energy-transducing ATPases and its association with the (alpha, beta, gamma) M. lysodeikticus ATPase complex appears to be dependent on bivalent cations. The present results do not preclude the possibility that the gamma subunit also plays a role in ATPase binding, in which, however, the major subunits do not seem to play a role.     

Effects of treatment with GABA(A) receptor subunit antisense oligodeoxynucleotides on GABA-stimulated 36Cl- influx in the rat cerebral cortex

GABA(A) receptor function was studied in cerebral cortical vesicles prepared from rats after intracerebroventricular microinjections of antisense oligodeoxynucleotides (aODNs) for alpha1, gamma2, beta1, beta2 subunits. GABA(A) receptor alpha1 subunit aODNs decreased alpha1 subunit mRNA by 59+/-10%. Specific [3H]GABA binding was decreased by alpha1 or beta2 subunit aODNs (to 63+/-3% and 64+/-9%, respectively) but not changed by gamma2 subunit aODNs (94+/-5%). Specific [3H]flunitrazepam binding was increased by alpha1 or beta2 subunit aODNs (122+/-8% and 126+/-11%, respectively) and decreased by gamma2 subunit aODNs (50+/-13%). The "knockdown" of specific subunits of the GABA(A )receptor significantly influenced GABA-stimulated 36Cl- influx. Injection of alpha1 subunit aODNs decreased basal 36Cl- influx and the GABA Emax; enhanced GABA modulation by diazepam; and decreased antagonism of GABA activity by bicuculline. Injection of gamma2 subunit aODNs increased the GABA Emax; reversed the modulatory efficacy of diazepam from enhancement to inhibition of GABA-stimulation; and reduced the antagonist effect of bicuculline. Injection of beta2 subunit aODNs reduced the effect of diazepam whereas treatment with beta1 subunit aODNs had no effect on the drugs studied. Conclusions from our studies are: (1) alpha1 subunits promote, beta2 subunits maintain, and gamma2 subunits suppress GABA stimulation of 36Cl- influx; (2) alpha1 subunits suppress, whereas beta2, and gamma2 subunits promote allosteric modulation by benzodiazepines; (3) diazepam can act as an agonist or inverse agonist depending on the relative composition of the receptor subunits: and (4) the mixed competitive/non-competitive effects of bicuculline result from activity at alpha1 and gamma2 subunits and the lack of activity at beta1 and beta2 subunits.     

Expression of functional G protein beta gamma dimers of defined subunit composition using a baculovirus expression system.

The 36-kDa beta 1, 35-kDa beta 2, and 6.5-kDa gamma 2 subunits of the heterotrimeric guanine nucleotide-binding proteins have been overexpressed in Sf9 cells using a baculovirus expression system. The gamma 2 subunit expressed in Sf9 cells incorporated label derived from [3H]mevalonate and is therefore likely to be isoprenylated, as is its mammalian counterpart. Extracts of Sf9 cells doubly infected with viruses encoding a beta subunit and viruses encoding a gamma subunit are active in promoting the pertussis toxin-catalyzed ADP-ribosylation of a G protein alpha subunit. However, extracts from Sf9 cells singly infected with viruses encoding either a beta or gamma subunit are not active in this assay. Results demonstrate utility of the insect/baculovirus system for expressing G protein beta gamma subunits of defined composition.     

Preferential assembly of epithelial sodium channel (ENaC) subunits in Xenopus oocytes: role of furin-mediated endogenous proteolysis

The epithelial sodium channel (ENaC) is preferentially assembled into heteromeric alphabetagamma complexes. The alpha and gamma (not beta) subunits undergo proteolytic cleavage by endogenous furin-like activity correlating with increased ENaC function. We identified full-length subunits and their fragments at the cell surface, as well as in the intracellular pool, for all homo- and heteromeric combinations (alpha, beta, gamma, alphabeta, alphagamma, betagamma, and alphabetagamma). We assayed corresponding channel function as amiloride-sensitive sodium transport (I(Na)). We varied furin-mediated proteolysis by mutating the P1 site in alpha and/or gamma subunit furin consensus cleavage sites (alpha(mut) and gamma(mut)). Our findings were as follows. (i) The beta subunit alone is not transported to the cell surface nor cleaved upon assembly with the alpha and/or gamma subunits. (ii) The alpha subunit alone (or in combination with beta and/or gamma) is efficiently transported to the cell surface; a surface-expressed 65-kDa alpha ENaC fragment is undetected in alpha(mut)betagamma, and I(Na) is decreased by 60%. (iii) The gamma subunit alone does not appear at the cell surface; gamma co-expressed with alpha reaches the surface but is not detectably cleaved; and gamma in alphabetagamma complexes appears mainly as a 76-kDa species in the surface pool. Although basal I(Na) of alphabetagamma(mut) was similar to alphabetagamma, gamma(mut) was not detectably cleaved at the cell surface. Thus, furin-mediated cleavage is not essential for participation of alpha and gamma in alphabetagamma heteromers. Basal I(Na) is reduced by preventing furin-mediated cleavage of the alpha, but not gamma, subunits. Residual current in the absence of furin-mediated proteolysis may be due to non-furin endogenous proteases.     

G alpha i-G alpha s chimeras define the function of alpha chain domains in control of G protein activation and beta gamma subunit complex interactions

Gs and Gi2 are G proteins whose alpha subunits are 65% homologous. Within the 355 amino acid alpha i2 polypeptide, substitution of residues Ile213-Lys319 with the corresponding alpha s region (Ile235-Arg356) generated a chimera that activated adenylyl cyclase, indicating that the alpha s activation domain resides within this 122 amino acid alpha s sequence. Mutation within alpha s residues Glu15-Pro144 resulted in an alpha s polypeptide having an enhanced rate of GDP dissociation. Mutation within two regions of the N-terminus influenced the ability of pertussis toxin to ADP-ribosylate the alpha subunit polypeptide, a reaction controlled by the beta gamma subunit complex. The findings define the G protein alpha subunit N-terminus as a regulatory region controlling beta gamma subunit interactions and GDP dissociation independent of the GTPase and effector activation domains.     

The amino terminus of G protein alpha subunits is required for interaction with beta gamma

The guanine nucleotide-binding proteins (G proteins), which transduce hormonal and light signals across the plasma membrane, are heterotrimers composed of alpha, beta, and gamma subunits. Activation of G proteins by guanine nucleotides is accompanied by dissociation of the heterotrimer: G + alpha.beta.gamma in equilibrium alpha G + beta.gamma. Brain contains several G proteins of which the most abundant are alpha 39.beta.gamma and alpha 41.beta.gamma. We have used proteolysis by trypsin to study the functional domains of the alpha subunits. In the presence of guanosine 5'-(3-O-thio)triphosphate, trypsin removes a 2-kDa peptide from the amino terminus of these proteins (Hurley, J. B., Simon, M. I., Teplow, D. B., Robishaw, J. D., and Gilman, A. G. (1984) Science 226, 860-862; Winslow, J. W., Van Amsterdam, J. R., and Neer, E. J. (1986) J. Biol. Chem. 261, 7571-7579). Tryptic cleavage does not affect the GTPase activity of the truncated molecule nor the apparent Km for GTP. However, removal of the 2-kDa amino-terminal peptide prevents association of the alpha subunits with beta.gamma. Since the apparent substrate for pertussis toxin-catalyzed ADP-ribosylation is the alpha.beta.gamma heterotrimer, the trypsin-cleaved alpha subunit is not a substrate for the toxin. Digestion of the carboxyl terminus of alpha 39 with carboxypeptidase A prevents ADP-ribosylation by pertussis toxin but does not interfere with the formation of alpha 39.beta.gamma heterotrimers. We do not yet know whether the amino-terminal region of alpha 39 interacts with beta gamma directly or whether it is necessary to maintain a conformation of alpha 39 which is required for heterotrimer formation. Further studies are needed to define the nature of the contracts between alpha and beta gamma subunits since understanding the structural basis for their reversible interaction is fundamental to understanding their function.     

Identification and characterization of a novel high-molecular-weight form of the integrin alpha 3 subunit.

The integrin alpha 3 beta 1 is a multiligand extracellular matrix receptor found on many cell types. Immunoprecipitations of 125I-surface-labeled prostate carcinoma cell lines, DU145 and PC-3, with the anti-alpha 3 integrin monoclonal antibodies J143 or PIB5, resulted in the coimmunoprecipitation, along with the expected alpha 3 beta 1 heterodimer, of a polypeptide with a molecular mass of 225 kDa. This protein could also be copurified with the 155-kDa alpha 3 and 115-kDa beta 1 subunits upon affinity chromatography of 125I-surface-labeled cell extracts on anti-alpha 3 antibody-Sepharose columns. Upon reduction, this 225-kDa protein generated 130- and 95-kDa polypeptides, while the 155-kDa alpha 3 subunit generated 130- and 25-kDa polypeptides. The 225-kDa protein did not generate a 25-kDa polypeptide. Deglycosylation and reduction of the 225-kDa protein resulted in the generation of 110- and 95-kDa polypeptides, while deglycosylation and reduction of the 155-kDa alpha 3 resulted in a 110-kDa polypeptide identical in size to the 110-kDa polypeptide generated from the 225-kDa protein. Peptide maps generated from the 110-kDa components of the 225-kDa polypeptide and the 155-kDa alpha 3 integrin subunit were identical, as were their N-terminal amino acid sequences. An antibody directed against the cytoplasmic domain of the alpha 3 subunit immunoprecipitated the 225-kDa polypeptide in addition to the 155-kDa alpha 3 subunit. Furthermore, Northern blot analysis of RNA from DU145 and PC-3 cells with a human alpha 3 cDNA probe identified an mRNA species of 6.2 kb in addition to a major mRNA species of 4.3 kb. The larger mRNA species, which is of an appropriate size for encoding a polypeptide of approximately 220-kDa, was not detectable in cells which did not express the 225-kDa protein. These data demonstrate that the 225-kDa polypeptide represents a novel integrin alpha 3 subunit consisting of the alpha 3 integrin heavy chain disulfide-bonded to a 95-kDa polypeptide which may represent an alternative "light" chain to the 25-kDa light chain of the alpha 3 subunit.     

Rhodopsin and the retinal G-protein distinguish among G-protein beta gamma subunit forms

The beta gamma subunits of G-proteins are composed of closely related beta 35 and beta 36 subunits tightly associated with diverse 6-10 kDa gamma subunits. We have developed a reconstitution assay using rhodopsin-catalyzed guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) binding to resolved alpha subunit of the retinal G-protein transducin (Gt alpha) to quantitate the activity of beta gamma proteins. Rhodopsin facilitates the exchange of GTP gamma S for GDP bound to Gt alpha beta gamma with a 60-fold higher apparent affinity than for Gt alpha alone. At limiting rhodopsin, G-protein-derived beta gamma subunits catalytically enhance the rate of GTP gamma S binding to resolved Gt alpha. The isolated beta gamma subunit of retinal G-protein (beta 1, gamma 1 genes) facilitates rhodopsin-catalyzed GTP gamma S exchange on Gt alpha in a concentration-dependent manner (K0.5 = 254 +/- 21 nM). Purified human placental beta 35 gamma, composed of beta 2 gene product and gamma-placenta protein (Evans, T., Fawzi, A., Fraser, E.D., Brown, L.M., and Northup, J.K. (1987) J. Biol. Chem. 262, 176-181), substitutes for Gt beta gamma reconstitution of rhodopsin with Gt alpha. However, human placental beta 35 gamma facilitates rhodopsin-catalyzed GTP gamma S exchange on Gt alpha with a higher apparent affinity than Gt beta gamma (K0.5 = 76 +/- 54 nM). As an alternative assay for these interactions, we have examined pertussis toxin-catalyzed ADP-ribosylation of the Gt alpha subunit which is markedly enhanced in rate by beta gamma subunits. Quantitative analyses of rates of pertussis modification reveal no differences in apparent affinity between Gt beta gamma and human placental beta 35 gamma (K0.5 values of 49 +/- 29 and 70 +/- 24 nM, respectively). Thus, the Gt alpha subunit alone does not distinguish among the beta gamma subunit forms. These results clearly show a high degree of functional homology among the beta 35 and beta 36 subunits of G-proteins for interaction with Gt alpha and rhodopsin, and establish a simple functional assay for the beta gamma subunits of G-proteins. Our data also suggest a specificity of recognition of beta gamma subunit forms which is dependent both on Gt alpha and rhodopsin. These results may indicate that the recently uncovered diversity in the expression of beta gamma subunit forms may complement the diversity of G alpha subunits in providing for specific receptor recognition of G-proteins.