Empty site forms of the SRP54 and SR alpha GTPases mediate targeting of ribosome-nascent chain complexes to the endoplasmic reticulum.

The SRP54 and SR alpha subunits of the signal recognition particle (SRP) and the SRP receptor (SR) undergo a tightly coupled GTPase cycle that mediates the signal sequence-dependent attachment of ribosomes to the Sec61 complex. Here, we show that SRP54 and SR alpha are in the empty site conformation prior to contact between the SRP-ribosome complex and the membrane-bound SR. Cooperative binding of GTP to SRP54 and SR alpha stabilizes the SRP-SR complex and initiates signal sequence transfer from SRP54 to Sec61 alpha. The GTP-bound conformations of SR alpha and SRP54 perform distinct roles, with SR alpha performing a predominant role in complex stabilization. Hydrolysis by both SRP54 and SR alpha is a prerequisite for dissociation of the SRP-SR complex.

The alpha subunit of the Saccharomyces cerevisiae oligosaccharyltransferase complex is essential for vegetative growth of yeast and is homologous to mammalian ribophorin I.

Oligosaccharyltransferase mediates the transfer of a preassembled high mannose oligosaccharide from a lipid-linked oligosaccharide donor to consensus glycosylation acceptor sites in newly synthesized proteins in the lumen of the rough endoplasmic reticulum. The Saccharomyces cerevisiae oligosaccharyltransferase is an oligomeric complex composed of six nonidentical subunits (alpha-zeta), two of which are glycoproteins (alpha and beta). The beta and delta subunits of the oligosaccharyltransferase are encoded by the WBP1 and SWP1 genes. Here we describe the functional characterization of the OST1 gene that encodes the alpha subunit of the oligosaccharyltransferase. Protein sequence analysis revealed a significant sequence identity between the Saccharomyces cerevisiae Ost1 protein and ribophorin I, a previously identified subunit of the mammalian oligosaccharyltransferase. A disruption of the OST1 locus was not tolerated in haploid yeast showing that expression of the Ost1 protein is essential for vegetative growth of yeast. An analysis of a series of conditional ost1 mutants demonstrated that defects in the Ost1 protein cause pleiotropic underglycosylation of soluble and membrane-bound glycoproteins at both the permissive and restrictive growth temperatures. Microsomal membranes isolated from ost1 mutant yeast showed marked reductions in the in vitro transfer of high mannose oligosaccharide from exogenous lipid-linked oligosaccharide to a glycosylation site acceptor tripeptide. Microsomal membranes isolated from the ost1 mutants contained elevated amounts of the Kar2 stress-response protein.

Signal sequence recognition and targeting of ribosomes to the endoplasmic reticulum by the signal recognition particle do not require GTP.

The identification of GTP-binding sites in the 54-kDa subunit of the signal recognition particle (SRP) and in both the alpha and beta subunits of the SRP receptor has complicated the task of defining the step in the protein translocation reaction that is controlled by the GTP-binding site in the SRP. Ribonucleotide binding assays show that the purified SRP can bind GDP or GTP. However, crosslinking experiments show that SRP54 can recognize the signal sequence of a nascent polypeptide in the absence of GTP. Targeting of SRP-ribosome-nascent polypeptide complexes, formed in the absence of GTP, to microsomal membranes likewise proceeds normally. To separate the GTPase cycles of SRP54 and the alpha subunit of the SRP receptor (SR alpha), we employed an SR alpha mutant that displays a markedly reduced affinity for GTP. We observed that the dissociation of SRP54 from the signal sequence and the insertion of the nascent polypeptide into the translocation site could only occur when GTP binding to SR alpha was permitted. These data suggest that the GTP binding and hydrolysis cycles of both SRP54 and SR alpha are initiated upon formation of the SRP-SRP receptor complex.

Protein translocation across the ER requires a functional GTP binding site in the alpha subunit of the signal recognition particle receptor.

The signal recognition particle (SRP)-mediated translocation of proteins across the RER is a GTP dependent process. Analysis of the primary amino acid sequence of one protein subunit of SRP (SRP54), as well as the alpha subunit of the SRP receptor (SR alpha), has indicated that these proteins contain predicted GTP binding sites. Several point mutations confined to the GTP binding consensus elements of SR alpha were constructed by site specific mutagenesis to define a role for the GTP binding site in SR alpha during protein translocation. The SR alpha mutants were analyzed using an in vitro system wherein SR alpha-deficient microsomal membranes were repopulated with SR alpha by in vitro translation of wild-type or mutant mRNA transcripts. SRP receptors containing SR alpha point mutants were analyzed for their ability to function in protein translocation and to form guanylyl-5'-imidodiphosphate (Gpp[NH]p) stabilized complexes with the SRP. Mutations in SR alpha produced SRP receptors that were either impaired or inactive in protein translocation. These SRP receptors were likewise unable to form Gpp(NH)p stabilized complexes with the SRP. One SR alpha point mutant, Thr 588 to Asn 588, required 50- to 100-fold higher concentrations of GTP relative to the wild-type SR alpha to function in protein translocation. This mutant has provided information on the reaction step in protein translocation that involves the GTP binding site in the alpha subunit of the SRP receptor.

Requirement of GTP hydrolysis for dissociation of the signal recognition particle from its receptor.

The signal recognition particle (SRP) directs signal sequence specific targeting of ribosomes to the rough endoplasmic reticulum. Displacement of the SRP from the signal sequence of a nascent polypeptide is a guanosine triphosphate (GTP)-dependent reaction mediated by the membrane-bound SRP receptor. A nonhydrolyzable GTP analog can replace GTP in the signal sequence displacement reaction, but the SRP then fails to dissociate from the membrane. Complexes of the SRP with its receptor containing the nonhydrolyzable analog are incompetent for subsequent rounds of protein translocation. Thus, vectorial targeting of ribosomes to the endoplasmic reticulum is controlled by a GTP hydrolysis cycle that regulates the affinity between the SRP, signal sequences, and the SRP receptor.

G-protein subunit mRNA levels in rat heart, liver, and adipose tissues: analysis by DNA-excess solution hybridization.

The steady-state levels of mRNAs for the G-proteins Gi alpha 2, Go alpha, and the G beta-subunits common to each were established in rat adipose, heart and liver. Uniformly-radiolabeled, single-stranded antisense probes were constructed from cDNAs or assembled from oligonucleotides. Direct comparison of the steady-state levels of the G-protein mRNAs was performed under identical assay conditions, and on a molar basis. In adipose, liver and heart, Gs alpha mRNA was more abundant than mRNA for Go alpha, Gi alpha, and G beta. In adipose tissue, mRNA levels were as follows: 19.4, 7.6, 7.0, and 2.3 amol mRNA per micrograms total cellular RNA for Gs alpha, G beta, Gi alpha 2, and Go alpha, respectively. In heart Gs alpha mRNA was less abundant than in adipose, but the relative trend among the G-protein subunits was the same. In liver, G beta mRNA was more abundant than either Go alpha or Gi alpha 2. Go alpha mRNA levels ranged from 1.2 to 2.3 amol/micrograms total RNA in liver and adipose, respectively. The present work demonstrates the many advantages of this strategy when applied to the study of a family of homologous, low-abundance proteins and establishes for the first time the molar levels of Gi alpha 2, Gs alpha, Go alpha, and G beta-subunit mRNAs in several mammalian tissues.

Regulation of transmembrane signalling elements: transcriptional, post-transcriptional and post-translational controls.

G-protein-mediated transmembrane signalling is a common motif in biology. The actions of a populous group of G-protein-linked receptors in hormone action, olfaction and vision in vertebrates are examples in which input signals are transferred from a receptor molecule (or photopigment) to an effector unit(s) via G-proteins. The expression and functional status of the receptors, G-proteins, and effectors that constitute these transmembrane signalling systems are regulated physiologically. Altering the abundance, function, or both of these elements provides the means for modulating transmembrane signalling and integration of information among separate pathways. Recent advances in the cell and molecular biology of transmembrane signalling elements provide insight as to the mechanisms by which regulation occurs. Transcriptional control is exemplified by glucocorticoid induction of beta-adrenergic receptor expression. Agonist-induced down-regulation of beta-adrenergic receptor mRNA via message destabilization best highlights post-transcriptional control. Examples of post-translational control of transmembrane signalling elements include protein phosphorylation, thioldisulphide exchange, and altered rates of protein degradation. Simultaneous analysis of physiological regulation at the levels of the gene, mRNA, and protein provide new opportunities for understanding how information processing extends from the plasma membrane to the genome.

G-protein mRNA levels during adipocyte differentiation.

G-protein-mediated transmembrane signaling in 3T3-L1 cells is modulated by differentiation. The regulation of G-protein expression in differentiating 3T3-L1 cells was probed at the level of mRNA by DNA-excess solution hybridization. Pertussis toxin-catalyzed ADP-ribosylation of G-protein alpha-subunits increased as fibroblasts differentiate to adipocytes. Steady-state levels of mRNA for Gi alpha 2 and Go alpha, in contrast, declined sharply. Immunoblotting with antipeptide antibodies specific for Gi alpha 2, too, revealed a decline in the steady-state expression of this pertussis toxin substrate. ADP-ribosylation of Gs alpha by cholera toxin was less in the adipocyte than fibroblast. Analysis by immunoblotting revealed only a modest decline in Gs alpha. Analysis of mRNA levels also demonstrated a decline for Gs alpha. mRNA levels for the G beta-subunits rose initially (25%) on day 1, declined from day 1 to day 3, and remained 25% lower in adipocytes than in fibroblasts. In 3T3-L1 adipocytes the molar amounts of subunit mRNAs were: 60.6 (Gs alpha); 2.1 (Gi alpha 2); and 1.5 (Go alpha) amol/microgram total cellular RNA. In rat fat cells these mRNA levels were 19.4 (Gs alpha); 7.0 (Gi alpha 2); and 2.3 (Go alpha). These data demonstrate that for Gi alpha 2 and Go alpha alike mRNA and protein expression decrease, not increase, in differentiation. A substrate for pertussis toxin other than Gi alpha 2 and Go alpha appears to be responsible for the increase in toxin-catalyzed labeling that accompanies differentiation of 3T3-L1 cells.

Thyroid hormones regulate G-protein beta-subunit mRNA expression in vivo.

Thyroid hormones exert "permissive effects" on the hormone-sensitive adenylate cyclase. Regulation of the expression of Gi (Gi alpha 2) and Gs by thyroid hormones in vivo was investigated at the level of mRNA. Steady-state levels of the mRNA for Gi alpha 2 and Gs alpha, as well as the G beta-subunits, were quantified using DNA excess solution hybridization analysis. Regulation of protein and mRNA expression in adipose tissue was investigated in hypothyroid, euthyroid, and hyperthyroid rats. In euthyroid animals, steady-state levels of mRNA (amol/microgram RNA) were 13.8, 5.9, and 5.7 for Gs alpha, Gi alpha 2, and G beta 1,2, respectively. Activation of adenylate cyclase by Gs is unaffected by thyroid status. Both Gs alpha and Gs alpha mRNA levels in hypothyroid rats were the same as those of controls (euthyroid). The inhibitory control of adenylate cyclase, in contrast, is markedly potentiated in hypothyroid rats. The expression of G1 alpha s and G beta-subunits was increased in hypothyroidism. Whereas Gi alpha 2 mRNA levels remained essentially unchanged, G beta 1,2 mRNA levels were observed to increase 45% in the hypothyroid state. In the hyperthyroid state G beta 1,2 mRNA levels were observed to decline by 35%. Regulation of G-protein subunit expression, at the level of mRNA, appears to be one component of permissive hormone action on transmembrane signalling.

Glucocorticoids modulate mRNA levels for G-protein beta-subunits.

Adrenalectomy decreases, whereas glucocorticoid treatment increases, the steady-state levels of G-protein beta-subunits (G beta) in rat fat-cells. A DNA-excess solution-hybridization assay was established to define the steady-state mRNA levels for G beta [5.8 +/- 0.4 amol/micrograms of RNA (n = 5) in control fat-cells]. G beta mRNA levels decrease by 20% after adrenalectomy; dexamethasone treatment reverses the decline. Dexamethasone treatment itself increases G beta mRNA levels by 50%.

Short-term hyperthyroidism modulates adenosine receptors and catalytic activity of adenylate cyclase in adipocytes.

The effects of short-term hyperthyroidism in vivo on the status of the components of the fat-cell hormone-sensitive adenylate cyclase were investigated. The number of beta-adrenergic receptors was elevated by about 25% in membranes of fat-cells isolated from hyperthyroid rats as compared with euthyroid rats, but their affinity for radioligand was unchanged. Membranes of hyperthyroid-rat fat-cells displayed less than 65% of the normal complement of receptors for [3H]cyclohexyladenosine. The affinity of the receptors for this ligand was normal. In contrast with the marked increase in the amounts of the alpha-subunits of the guanine nucleotide-binding proteins Gi (Mr 41,000) and Go (Mr 39,000) observed in the hypothyroid state [Malbon, Rapiejko & Mangano (1985) J. Biol. Chem. 260, 2558-2564], the amounts of alpha-Gi, alpha-Go as well as alpha-Gs subunits [Mr 42,000 (major) and 46,000/48,000 (minor)] were not changed by hyperthyroidism. Adenylate cyclase activity in response to forskolin, guanosine 5'-[gamma-thio]triphosphate or isoprenaline, in contrast, was decreased by 30-50% in fat-cell membranes from hyperthyroid rats. Fat-cells isolated from hyperthyroid rats accumulated cyclic AMP to less than 50% of the extent in their euthyroid counterparts in the presence of adenosine deaminase and either adrenaline or forskolin, suggesting a decrease in the amount or activity of the catalytic subunit of adenylate cyclase. In the absence of exogenous adenosine deaminase, cyclic AMP accumulation in response to adrenaline was elevated rather than decreased in fat-cells from hyperthyroid rats. The inhibitory influence of adenosine is apparently limited in the hyperthyroid state by the decreased complement of inhibitory R-site purinergic receptors in these fat-cells. Short-term hyperthyroidism modulates the fat-cell adenylate cyclase system at the receptor level (beta-receptor number increased, R-site purinergic-receptor number decreased) and the catalytic subunit of adenylate cyclase.

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.

Fat cell adenylate cyclase system. Enhanced inhibition by adenosine and GTP in the hypothyroid rat.

Hypothyroidism is associated with an enhanced sensitivity of rat fat cells to the inhibitory action of adenosine and adenosine agonists. The sensitivity of the forskolin-stimulated cyclic AMP response of rat fat cells to the adenosine agonist N6-phenylisopropyladenosine is amplified 3-fold by hypothyroidism. Forskolin-stimulated adenylate cyclase activity is more sensitive to inhibition by this adenosine agonist in membranes of fat cells isolated from hypothyroid as compared to euthyroid rats. Hypothyroidism does not significantly alter the number of affinity of binding sites for N6-cyclohexyl[3H]adenosine or N6-phenylisopropyladenosine in membranes of rat fat cells. GTP-induced inhibition of forskolin-stimulated adenylate cyclase was markedly enhanced in the hypothyroid state, suggesting an alteration in the inhibitory regulatory component (Ni)-mediated control of adenylate cyclase. Incubating membranes with [alpha-32P]NAD+ and preactivated pertussis toxin results in the radiolabeling of two peptides with Mr = 40,000 and 41,000 as visualized in autoradiograms of polyacrylamide gels run in sodium dodecyl sulfate. The amount of label incorporated by pertussis toxin into these two peptides (putative subunits of Ni) per mg of protein of membrane is increased 2-3-fold in the hypothyroid state. The amount of the stimulatory regulatory component, Ns, in fat cell membranes is not altered by hypothyroidism (Malbon, C. C., Graziano, M. P., and Johnson, G. L. (1984) J. Biol. Chem. 259, 3254-3260). The amplified response of hypothyroid rat fat cells to the inhibitory action of adenosine appears to reflect a specific increase in the activity and abundance of Ni.

Pertussis toxin catalyzes the ADP-ribosylation of two distinct peptides, 40 and 41 kDa, in rat fat cell membranes.

Pertussis toxin catalyzes the ADP-ribosylation of a single 41-kDa peptide of membranes prepared from rat hepatocytes, S49 mouse lymphoma wild-type and cyc-mutant cells. This 41-kDa peptide has been shown to be the alpha-subunit of the inhibitory, guanine nucleotide binding regulatory component of adenylate cyclase (Ni). Incubating membranes of rat fat cells with pertussis toxin and [32P]NAD+ radiolabels a 41- and a 40-kDa peptide. Possible homologies between these peptides were investigated by comparing the electrophoretic patterns of proteolytic fragments derived from each of them that are radiolabeled by [32P]NAD+ and pertussis toxin. The 40-kDa substrate for pertussis toxin-catalyzed ADP-ribosylation and the alpha-subunit of Ni in rat fat cells appear to be homologous, but non-identical peptides.