Role of gag sequence in the biochemical properties and transforming activity of the avian sarcoma virus UR2-encoded gag-ros fusion protein.

The transforming protein P68gag-ros of avian sarcoma virus UR2 is a transmembrane tyrosine protein kinase molecule with the gag portion protruding extracellularly. To investigate the role of the gag moiety in the biochemical properties and biological functions of the P68gag-ros fusion protein, retroviruses containing the ros coding sequence of UR2 were constructed and analyzed. The gag-free ros protein was expressed from one of the mutant retroviruses at a level 10 to 50% of that of the wild-type UR2. However, the gag-free ros-containing viruses were not able to either transform chicken embryo fibroblasts or induce tumors in chickens. The specific tyrosine protein kinase activity of gag-free ros protein is about 10- to 20-fold reduced as judged by in vitro autophosphorylation. The gag-free ros protein is still capable of associating with membrane fractions including the plasma membrane, indicating that sequences essential for recognition and binding membranes must be located within ros. Upon passages of the gag-free mutants, transforming and tumorigenic variants occasionally emerged. The variants were found to have regained the gag sequence fused to the 5' end of the ros, apparently via recombination with the helper virus or through intramolecular recombination between ros and upstream gag sequences in the same virus construct. All three variants analyzed code for gag-ros fusion protein larger than 68 kDa. The gag-ros recombination junction of one of the transforming variants was sequenced and found to consist of a p19-p10-p27-ros fusion sequence. We conclude that the gag sequence is essential for the transforming activity of P68gag-ros but is not important for its membrane association.     

Protein domains,catalytic activity,and subcellular distribution of neuropathy target esterase in Mammalian cells

Neuropathy target esterase (NTE), the human homologue of a protein required for brain development in Drosophila, has a predicted amino-terminal transmembrane helix (TM), a putative regulatory (R) domain, and a hydrophobic catalytic (C) domain. Here we describe the expression, in COS cells, of green fluorescent protein-tagged constructs of NTE and mutant proteins lacking the TM or the R- or C-domains. Esterase assays and Western blots of particulate and soluble fractions indicated that neither the TM nor R-domain is essential for NTE catalytic activity but that this activity requires membrane association to which the TM, R-, and C-domains all contribute. Experiments involving proteinase treatment revealed that most of the NTE molecule is exposed on the cytoplasmic face of membranes. In cells expressing a moderate level of NTE and all cells expressing DeltaC-NTE, fluorescence was distributed in an endoplasmic reticulum (ER)-like pattern. Cells expressing high levels of NTE showed aberrant distribution of ER marker proteins and accumulation of NTE on the cytoplasmic surface of ER-derived tubuloreticular aggregates. Deformation of the ER was also seen in cells expressing DeltaR-NTE or enzymatically inactive S966A-NTE but not DeltaTM-NTE. The data suggest that NTE is anchored in the ER via its TM, that its R- and C-domains also interact with the cytoplasmic face of the ER, and that overexpression of NTE causes ER aggregation via intermolecular association of its C-domains.     

Cell surface expression of v-fms-coded glycoproteins is required for transformation.

The viral oncogene v-fms encodes a transforming glycoprotein with in vitro tyrosine-specific protein kinase activity. Although most v-fms-coded molecules remain internally sequestered in transformed cells, a minor population of molecules is transported to the cell surface. An engineered deletion mutant lacking 348 base pairs of the 3.0-kilobase-pair v-fms gene encoded a polypeptide that was 15 kilodaltons smaller than the wild-type v-fms gene product. The in-frame deletion of 116 amino acids was adjacent to the transmembrane anchor peptide located near the middle of the predicted protein sequence and 432 amino acids from the carboxyl terminus. The mutant polypeptide acquired N-linked oligosaccharide chains, was proteolytically processed in a manner similar to the wild-type glycoprotein, and exhibited an associated tyrosine-specific protein kinase activity in vitro. However, the N-linked oligosaccharides of the mutant glycoprotein were not processed to complex carbohydrate chains, and the glycoprotein was not detected at the cell surface. Cells expressing high levels of the mutant glycoprotein did not undergo morphological transformation and did not form colonies in semisolid medium. The transforming activity of the v-fms gene product therefore appears to be mediated through target molecules on the plasma membrane.     

Functional implications of post-translational modifications of phospholipases D1 and D2

Our previous studies showed that truncation of the N-terminal 168 amino acids of rat brain phospholipase D1 (rPLD1) abolishes its response to protein kinase C (PKC) and greatly diminishes its palmitoylation and Ser/Thr phosphorylation. In this study, we show that the response to PKC as well as the palmitoylation and Ser/Thr phosphorylation were restored when the truncated rPLD1 mutant (rPLD1(DeltaN168)) was coexpressed with a fragment containing the N-terminal 168 amino acids. Immunoprecipitation experiments showed that the N-terminal fragment associated with rPLD1(DeltaN168) when coexpressed in COS 7 cells and that palmitoylation of Cys(240) and Cys(241) was not necessary for the association. In addition, we found that rat PLD2 (rPLD2) was palmitoylated on Cys(223) and Cys(224) in COS 7 cells. Mutation of both these cysteines reduced the basal activity of rPLD2, however its response to PMA stimulation in vivo was retained. As in the case of rPLD1, loss of palmitoylation weakened membrane association of rPLD2. In summary, the N-terminal 168-amino-acid fragment of rPLD1 can associate with truncated rPLD1(DeltaN168) to restore its palmitoylation, Ser/Thr phosphorylation and PKC response. Although rPLD2 differs from rPLD1 in many properties, it is palmitoylated at the corresponding conserved cysteine residues in COS 7 cells.     

Transformation of chicken embryo fibroblast cells by avian retroviruses containing the human Fyn gene and its mutated genes.

The transforming activity of the human fyn protein, p59fyn, which is a kinase of the src family, was investigated by testing the effect of recombinant avian retrovirus (Fyn virus) expressing p59fyn on chickens or cultured chicken embryo fibroblast (CEF) cells. The Fyn virus did not induce transformed foci. After several passages of the virus stock on CEF cells, however, a few foci were detected in the presence of dimethyl sulfoxide. Chickens inoculated with Fyn virus at the stage of 12-day-old embryos developed fibrosarcomas 3 to 6 weeks after hatching. The viruses obtained from these foci and from one of the tumor tissues showed high transforming activity in the presence of dimethyl sulfoxide, suggesting that these viruses carry spontaneous mutations of the fyn gene. Four fyn genes from CEF DNAs infected with transforming viruses were molecularly cloned, and their products were confirmed to possess transforming activity. DNA sequence analysis of the fyn genes showed that two of the four mutants have Thr instead of Ile at position 338 in the kinase domain. The other two mutants carry deletions of 78 and 108 base pairs, respectively, which result in complete loss of region C of SH2. The overall level of proteins containing phosphotyrosine was significantly higher in transformed cells than in normal CEF cells. Our data indicate that when expressed at high levels in a retrovirus, normal p59fyn cannot cause cellular transformation, but that mutant p59fyn with either a single amino acid substitution in the kinase domain or a deletion including region C produces a transforming protein, perhaps due to enhanced tyrosine kinase activity. This is the first observation that deletion of region C can unmask the potential transforming activity of a src family kinase.     

Molecular and biochemical bases for activation of the transforming potential of the proto-oncogene c-ros.

The transforming gene of avian sarcoma virus UR2, v-ros, encodes a receptor-like protein tyrosine kinase and differs from its proto-oncogene, c-ros, in its 5' truncation and fusion to viral gag, a three-amino-acid (aa) insertion in the transmembrane (TM) domain, and changes in the carboxyl region. To explore the basis for activation of the c-ros transforming potential, various c-ros retroviral vectors containing those changes were constructed and studied for their biological and biochemical properties. Ufcros codes for the full-length c-ros protein of 2,311 aa, Uppcros has 1,661-aa internal deletion in the extracellular domain, CCros contains the 3' c-ros cDNA fused 150 aa upstream of the TM domain to the UR2 gag, CVros is the same as CCros except that the 3' region is replaced by that of v-ros, and VCros is the same as CCros except that the 5' region is replaced by that of v-ros. The Ufcros, Uppcros, CCros, and CVros are inactive in transforming chicken embryo fibroblasts, whereas VCros is as potent as UR2 in cell-transforming and tumorigenic activities. Upon passages of CCros and CVros viruses, the additional extracellular sequence in comparison with that of v-ros was delected; concurrently, both viruses (named CC5d and CV5d, respectively) attained moderate transforming activity, albeit significantly lower than that of UR2 or VCros. The native c-ros protein has a very low protein tyrosine kinase activity, whereas the ppcros protein is constitutively activated in kinase activity. The inability of CCros and CVros to transform chicken embryo fibroblasts is consistent with the inefficient membrane association, instability, and low kinase activity of their encoded proteins. The CC5d and CV5d proteins are indistinguishable in kinase activity, membrane association, and stability from the v-ros protein. The reduced transforming potency of CC5d and CV5d proteins can be attributed only to their differential substrate interaction, notably the failure to phosphorylate a 88-kDa protein. We conclude that the 5' rather than the 3' modification of c-ros is essential for its oncogenic activation; the sequence upstream of the TM domain has a negative effect on the transforming activity of CCros and CVros and needs to be deleted to activate their biological activity.     

Molecular genetic analysis of the regulatory and catalytic domains of protein kinase C

We have constructed the expression plasmids harboring protein kinase C (PKC) mutant cDNAs with a series of deletions in the PKC coding region. These plasmids were transfected into COS7 cells to characterize the PKC mutants. Immunoblot analysis using the anti-PKC antibody identified proteins with the Mr values expected from the PKC mutant cDNAs in the extracts from COS7 cells. The wild-type PKC, when expressed in COS7 cells, conferred increased phorbol ester binding activity on intact cells; but the PKC mutants with the deletion around the C1 region did not show this activity. The wild-type PKC showed protein kinase activity dependent on phospholipid, Ca2+, and phorbol ester, whereas these PKC mutants exhibited protein kinase activity independent of the activators in a cell-free system. A PKC mutant cDNA with the deletion in the C2 region gave increased phorbol ester binding activity. Protein kinase activity of this mutant was much less dependent on Ca2+ compared with the wild-type PKC. A PKC mutant cDNA with the deletion in the C3 region conferred increased phorbol ester binding activity, but neither activator-dependent nor -independent protein kinase activity. These results indicate that elimination of the C1 region of PKC gives rise to constitutively active PKC independent of phospholipid, Ca2+, and phorbol ester and that the C1-C3 regions play distinct roles in the regulatory and catalytic function of PKC. In another series of experiments, transfection of some PKC mutant cDNAs with the deletions around the C1 region into Chinese hamster ovary and Jurkat cells activated the activator protein-1-binding element or the c-fos gene enhancer linked to the chloramphenicol acetyltransferase reporter gene in the absence of phorbol ester. Microinjection of these constructs into Xenopus oocytes induced initiation of germinal vesicle breakdown, indicating that they stimulated the PKC pathway in vivo. Thus, the phorbol ester-independent PKC mutant cDNAs could be a powerful tool to investigate the transmembrane signaling pathway mediated by PKC.     

The absence of myristic acid decreases membrane binding of p60src but does not affect tyrosine protein kinase activity.

We have constructed two point mutants of Rous sarcoma virus in which the amino-terminal glycine residue of the transforming protein, p60src, was changed to an alanine or a glutamic acid residue. Both mutant proteins failed to become myristylated and, more importantly, no longer transformed cells. The lack of transformation could not be attributed to defects in the catalytic activity of the mutant p60src proteins. In vitro phosphorylation of the peptide angiotensin or of the cellular substrate proteins enolase and p36 revealed no significant differences in the Km or specific activity of the mutant and wild-type p60src proteins. However, when cellular fractions were prepared, less than 12% of the nonmyristylated p60src proteins was bound to membranes. In contrast, more than 82% of the wild-type protein was associated with membranes. Wild-type p60src was phosphorylated by protein kinase C, a protein kinase which associates with membranes when activated. The mutant proteins were not. This finding supports the idea that within the intact cell the nonmyristylated p60src proteins are cytoplasmic and suggests that this apparent solubility is not an artifact of the cell fractionation procedure. The myristyl groups of p60src apparently encourages a tight association between protein and membranes and, by determining the cellular location of the enzyme, allows transformation to occur.     

Membrane association of the transforming protein of avian sarcoma virus UR2 and mutants temperature sensitive for cellular transformation and protein kinase activity.

The localization of the transforming protein P68gag-ros of avian sarcoma virus UR2, which has a hydrophobic region at the N terminus of its ros-specific tyrosine kinase-encoding sequence, was examined by subcellular fractionation. P68 behaved as an integral membrane protein associated with the plasma membrane of transformed cells. P68 became membrane associated very rapidly in its biogenesis. Three temperature-sensitive mutants of UR2 were isolated and characterized. Cells infected with the mutants were temperature sensitive for morphological alteration and colony formation. The mutant P68 proteins were membrane associated in mutant-infected cells regardless of the temperature but were active as protein kinases only at the permissive temperature. The results suggest that P68 is a membrane-associated protein whose kinase activity plays a crucial role in UR2-mediated cell transformation.     

GALLEX,a measurement of heterologous association of transmembrane helices in a biological membrane

Whereas a variety of two-hybrid systems are available to measure the interaction of soluble proteins, related methods are significantly less developed for the measurement of membrane protein interactions. Here we present a two-hybrid system to follow the heterodimerization of membrane proteins in the Escherichia coli inner membrane. The method is based on the repression of a reporter gene activity by two LexA DNA binding domains with different DNA binding specificities. When coupled to transmembrane domains, heterodimeric association is reported by repression of beta-galactosidase synthesis. The LexA-transmembrane chimeric proteins were found to correctly insert into the membrane, and reproducible signals were obtained measuring the homodimerization as well as heterodimerization of wild-type and mutant glycophorin A transmembrane helices. The GALLEX data were compared with data recently gained by other methods and discussed in the general context of heteroassociation of single TM helices. Additionally, the formation of heterodimers between the TM domains of the alpha(4) and the beta(7) integrin subunits were tested. The results show that both homo- and heterodimerization of membrane proteins can be measured accurately using the GALLEX system.     

Determination of the topology of the hydrophobic segment of mammalian diacylglycerol kinase epsilon in a cell membrane and its relationship to predictions from modeling

The epsilon isoform of diacylglycerol kinase (DGKepsilon) is unique among mammalian DGKs in having a segment of hydrophobic amino acids comprising approximately residues 20 to 41. Several algorithms predict this segment to be a transmembrane (TM) helix. Using PepLook, we have performed an in silico analysis of the conformational preference of the segment in a hydrophobic environment comprising residues 18 to 42 of DGKepsilon. We find that there are two distinct groups of stable conformations, one corresponding to a straight helix that would traverse the membrane and the second corresponding to a bent helix that would enter and leave the same side of the membrane. Furthermore, the calculations predict that substituting the Pro32 residue in the hydrophobic segment with an Ala will cause the hydrophobic segment to favor a TM orientation. We have expressed the P32A mutant of DGKepsilon, with a FLAG tag (an N-terminal 3xFLAG epitope tag) at the amino terminus, in COS-7 cells. We find that this mutation causes a large reduction in both k(cat) and K(m) while maintaining k(cat)/K(m) constant. Specificity of the P32A mutant for substrates with polyunsaturated acyl chains is retained. The P32A mutant also has higher affinity for membranes since it is more difficult to extract from the membrane with high salt concentration or high pH compared with the wild-type DGKepsilon. We also evaluated the topology of the proteins with confocal immunofluorescence microscopy using NIH 3T3 cells. We find that the FLAG tag at the amino terminus of the wild-type enzyme is not reactive with antibodies unless the cell membrane is permeabilized with detergent. We also demonstrate that at least a fraction of the wild-type DGKepsilon is present in the plasma membrane and that comparable amounts of the wild-type and P32A mutant proteins are in the plasma membrane fraction. This indicates that in these cells the hydrophobic segment of the wild-type DGKepsilon is not TM but takes up a bent conformation. In contrast, the FLAG tag at the amino terminus of the P32A mutant is exposed to antibody both before and after membrane permeabilization. This modeling approach thus provides an explanation, not provided by simple predictive algorithms, for the observed topology of this protein in cell membranes. The work also demonstrates that the wild-type DGKepsilon is a monotopic protein.     

The transmembrane helical segment but not the invariant lysine is required for the kinase activity of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10).

The large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) is a chimera consisting, at the amino terminus, of a Ser/Thr protein kinase (PK) with features of a signal peptide and a transmembrane (TM) helical segment, and at the carboxy-terminus, of the ribonucleotide reductase (Chung et al., 1989, 1990). Membrane immunofluorescence of ICP10 transformed cells with antibodies to synthetic peptides located upstream or downstream of the TM indicates that ICP10 is a membrane-spanning protein. Site-directed and deletion mutants were used to further characterize ICP10-PK. Mutation of Gly106 in catalytic motif I or of the invariant Lys in catalytic motif II, and deletion of both motifs (amino acids 106-178) did not eliminate kinase activity. PK activity was retained by the invariant Lys mutant expressed in bacteria and following protein separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transfer to membrane filters. Both ICP10 and the invariant Lys mutant bound 14C-labeled rho-fluorosulfonylbenzoyl 5'-adenosine, an ATP affinity analog. The deletion mutant had 4-fold lower kinase activity than ICP10-PK, and it was insensitive to Mn2+, suggesting that these motifs are involved in Mn2+ activation of kinase activity. PK activity was lost by deletion of the TM segment (amino acid residues 85-106).     

Distinct membrane localization and kinase association of the two isoforms of CD58

The adhesion molecule CD58 is involved in intercellular adhesion and in signal transduction. It is natively expressed in both a transmembrane form and a glycosylphosphatidylinositol (GPI)-anchored form, and hence provides a model for the study of two distinct membrane-anchored forms of the same protein in the same cell. We demonstrate here that the two isoforms of CD58 are localized in distinct membrane compartments. The GPI-anchored form localizes in lipid rafts, while the transmembrane form resides in nonraft domains. In addition to distinct membrane localization, the two isoforms of CD58 differ in their association with protein kinases. GPI-anchored CD58, residing in raft domains, is constitutively associated with protein kinases. However, cross-linking mediates a substantial increase in kinase activity which is predominantly associated with the transmembrane CD58 in nonraft membrane domains. The extensive inducible kinase activity, associated with transmembrane CD58, is demonstrated in wild-type cells as well as in GPI-deficient variant cells. Thus, although the transmembrane CD58 is excluded from rafts, it may trigger signaling independently of the GPI-linked isoform.     

Isoprenylation of an inhibitory G protein alpha subunit occurs only upon mutagenesis of the carboxyl terminus

We transfected COS cells with expression vectors for the wild-type G protein alpha i1 subunit (pWT) and for mutated alpha i1 subunits, including the nonmyristylated glycine 2 to alanine mutant (pGA) and mutants in which the carboxyl termini of pWT and pGA were changed from CGLF to CVLS (pCVLS and pGA-CVLS, respectively). Immunoblot analysis of transfected COS cells with an antibody to residues 159-168 of the alpha i1 protein indicated that all four proteins were expressed. Unlike the WT and GA proteins, both CVLS mutant proteins failed to react with an antibody specific for the carboxyl terminus and failed to undergo pertussis toxin-catalyzed ADP-ribosylation. Analysis of COS cell lysates after [3H]mevalonic acid labeling indicated that specific incorporation of radioactivity occurred only in the alpha i1 subunits with the CVLS mutation. Immuno-precipitation of COS cell fractions after labeling with [35S]methionine indicated that both WT and CVLS mutant proteins were localized predominantly in the particulate fraction, whereas GA and GA-CVLS mutant proteins were found primarily in the soluble fraction. These results directly demonstrate that the carboxyl-terminal sequence, CGLF, is incapable of leading to isoprenylation but that alteration of two residues (glycine to valine, phenylalanine to serine) is sufficient to promote isoprenylation.     

The transmembrane domain sequence affects the structure and function of the Newcastle disease virus fusion protein

The role of specific sequences in the transmembrane (TM) domain of Newcastle disease virus (NDV) fusion (F) protein in the structure and function of this protein was assessed by replacing this domain with the F protein TM domains from two other paramyxoviruses, Sendai virus (SV) and measles virus (MV), or the TM domain of the unrelated glycoprotein (G) of vesicular stomatitis virus (VSV). Mutant proteins with the SV or MV F protein TM domains were expressed, transported to cell surfaces, and proteolytically cleaved at levels comparable to that of the wild-type protein, while mutant proteins with the VSV G protein TM domain were less efficiently expressed on cell surfaces and proteolytically cleaved. All mutant proteins were defective in all steps of membrane fusion, including hemifusion. In contrast to the wild-type protein, the mutant proteins did not form detectable complexes with the NDV hemagglutinin-neuraminidase (HN) protein. As determined by binding of conformation-sensitive antibodies, the conformations of the ectodomains of the mutant proteins were altered. These results show that the specific sequence of the TM domain of the NDV F protein is important for the conformation of the preactivation form of the ectodomain, the interactions of the protein with HN protein, and fusion activity.     

Product of in vitro translation of the Rous sarcoma virus src gene has protein kinase activity.

In vitro translation of Rous sarcoma virus virion RNA resulted in the synthesis of a protein kinase which, when immunoprecipitated with antitumor serum, phosphorylated the immunoglobulin heavy chain. Even though in vitro translation of virion RNA resulted in the synthesis of a number of polypeptides which were recognized by antitumor serum, control experiments demonstrated that an immunoprecipitable protein kinase activity was found only when an immunoprecipitable p60src, the polypeptide product of the src gene, was synthesized. A protein kinase with similar properties was therefore intimately associated with p60src which was synthesized in vitro in the reticulocyte lysate, just as it is with p60src which is obtained from transformed chick and mammalian cells. It is therefore highly unlikely that this association is artifactual. ts NY68 is a mutant of Rous sarcoma virus which is able to transform cells at 36 but not at 41 degrees C. In vitro translation of ts NY68 virion RNA at 30 degrees C resulted in efficient synthesis of immunoprecipitable p60src, but very inefficient synthesis of an immunoprecipitable protein kinase. The p60src obtained by in vitro translation of wild-type virion RNA was more than 20-fold more active as a protein kinase than was that obtained from ts NY68 RNA. The correlation in the case of ts NY68 of a deficiency in protein kinase activity with an inability to transform cells at high temperature suggests that the protein kinase activity associated with p60src is indeed critical to cellular transformation.     

Proteomic Analysis of Rice Plasma Membrane-associated Proteins in Response to Chitooligosaccharide Elicitors

Chitooligomers or chitooligosaccharides （COS） are elicitors that bind to the plasma membrane （PM） and elicit various defense responses. However, the PM-bound proteins involved in elicitor-mediated plant defense responses still remain widely unknown. In order to get more information about PM proteins involved in rice defense responses, we conducted PM proteomic analysis of the rice suspension cells elicited by COS. A total of 14 up- or downregulated protein spots were observed on 2-D gels of PM fractions at 12 h and 24 h after COS incubation. Of them, eight protein spots were successfully identified by MS （mass spectrography） and predicted to be associated to the PM and function in plant defense, including a putative PKN/PRK1 protein kinase, a putative pyruvate kinase isozyme G, a putative zinc finger protein, a putative MAR-binding protein MFP1, and a putative calcium-dependent protein kinase. Interestingly, a COS-induced pM5-like protein was identified for the first time in plants, which is a transmembrane nodal modulator in transforming growth factor-β（TGFβ） signaling in vertebrates. We also identified two members of a rice polyprotein family, which were up-regulated by COS. Our study would provide a starting point for functionality of PM proteins in the rice basal defense.     

Transmembrane signal transduction by the Escherichia coli osmotic sensor, EnvZ: intermolecular complementation of transmembrane signalling

Summary
The Escherichia coli regulatory proteins, EnvZ and OmpR, are crucially involved in expression of the outer membrane proteins OmpF/OmpC in response to the medium osmolarity. The EnvZ protein is presumably a membrane-located osmotic sensor (or signal transducer), which exhibits both kinase and phosphatase activities specific for the OmpR protein. To examine the functional importance of the membrane-spanning segments (named TM1 and TM2) of EnvZ molecules in transmembrane signalling, a set of EnvZ mutants, each having amino acid substitutions within the membrane-spanning regions, was characterized in terms of both their in vivo phenotype and in vitro catalytic activities. One of them, characterized further, has an amino acid change (Pro-41 to Ser or Leu) In TM1, and appeared to be defective in its phosphatase activity but not in its kinase activity. This EnvZ mutant conferred a phenotype of OmpF⁻/OmpC-constitutive. For this EnvZ(P41S or P41L) mutant, a set of intragenic suppressors, each exhibiting a wild-type phenotype of OmpF⁺/OmpC⁺, was isolated. These suppresor mutants were revealed to have an additional amino acid change within either TM1 or TM2. Furthermore, they exhibited restored phosphatase activity (i.e., both kinase⁺ and phosphatase⁺ activities). It was further demonstrated that one of the suppressors, EnvZ(Arg-180 to Trp in TM2), was able to suppress the defects in both the in vivo phenotype and the in vitro catalytic activities caused by EnvZ(P41S), through intermolecular complementation. These results are best interpreted as meaning that an intimate intermolecular interaction between the membrane–spanning segments of EnvZ is crucial for transmembrane signalling per se in response to an external osmotic stimulus.     

Influence of gamma subunit prenylation on association of guanine nucleotide-binding regulatory proteins with membranes.

Two approaches were taken to address the possible role of gamma-subunit prenylation in dictating the cellular distribution of guanine nucleotide-binding regulatory proteins. Prenylation of gamma subunits was prevented by site-directed mutagenesis or by inhibiting the synthesis of mevalonate, the precursor of cellular isoprenoids. When beta or gamma subunits were transiently expressed in COS-M6 simian kidney cells (COS) cells, the proteins were found in the membrane fraction by immunoblotting. Immunofluorescence experiments indicated that the proteins were distributed to intracellular structures in addition to plasma membranes. Replacement of Cys68 of gamma with Ser prevented prenylation of the mutant protein and association of the protein with the membrane fraction of COS cells. Immunoblotting results demonstrated that some of the beta subunits were found in the cytoplasm when coexpressed with the nonprenylated mutant gamma subunit. When Neuro 2A cells were treated with compactin to inhibit protein prenylation, a fraction of endogenous beta and gamma was distributed in the cytoplasm. It is concluded that prenylation facilitates association of gamma subunits with membranes, that the cellular location of gamma influences the distribution of beta, and that prenylation is not an absolute requirement for interaction of beta and gamma.