Deletion within the amino-terminal region of Gs alpha impairs its ability to interact with beta gamma subunits and to activate adenylate cyclase.

Proteolytic experiments performed on transducin and Go alpha subunit strongly suggest that the amino-terminal residues of the alpha chain are involved in the interaction with beta gamma subunits. To test the possibility that the same region in Gs may fulfill a similar function, we introduced a deletion in the amino-terminal domain of Gs alpha. The properties of the wild type and the deleted alpha chains were characterized on in vitro translated proteins or after reconstitution of cyc- membranes by in vitro-translated alpha subunits. The mutant (delta 2-29) Gs alpha could still bind guanosine 5'-3-O-(thio)triphosphate, as revealed by its resistance to trypsin proteolysis and was still able to interact with the membrane. However, (delta 2-29) Gs alpha was not ADP-ribosylated by cholera toxin. In contrast to Gs alpha, addition of beta gamma subunits did not increase the rate of sedimentation of (delta 2-29) Gs alpha in sucrose gradients. Binding experiments on reconstituted membranes showed that the coupling to beta-adrenergic receptors was very low with (delta 2-29) Gs alpha. Finally, the mutant did not restore activation of adenylate cyclase of cyc- membranes. We propose that the primary functional defect is the loss of interaction with beta gamma subunits, which secondarily impairs beta gamma-dependent properties such as receptor coupling and cholera toxin-catalyzed ADP-ribosylation. However, it remains to be established that the lack of adenylate cyclase activation also results from this impaired interaction with beta gamma subunits.     

The carboxy-terminal domain of Gs alpha is necessary for anchorage of the activated form in the plasma membrane

GTP-binding proteins which participate in signal transduction share a common heterotrimeric structure of the alpha beta gamma-type. In the activated state, the alpha subunit dissociates from the beta gamma complex but remains anchored in the membrane. The alpha subunits of several GTP-binding proteins, such as Go and Gi, are myristoylated at the amino terminus (Buss, J. E., S. M. Mumby, P. J. Casey, A. G. Gilman, and B. M. Sefton. 1987. Proc. Natl. Acad. Sci. USA. 84:7493-7497). This hydrophobic modification is crucial for their membrane attachment. The absence of fatty acid on the alpha subunit of Gs (Gs alpha), the protein involved in adenylate cyclase activation, suggests a different mode of anchorage. To characterize the anchoring domain of Gs alpha, we used a reconstitution model in which posttranslational addition of in vitro-translated Gs alpha to cyc- membranes (obtained from a mutant of S49 cell line which does not express Gs alpha) restores the coupling between the beta-adrenergic receptor and adenylate cyclase. The consequence of deletions generated by proteolytic removal of amino acid sequences or introduced by genetic removal of coding sequences was determined by analyzing membrane association of the proteolyzed or mutated alpha chains. Proteolytic removal of a 9-kD amino-terminal domain or genetic deletion of 28 amino-terminal amino acids did not modify the anchorage of Gs alpha whereas proteolytic removal of a 1-kD carboxyterminal domain abolished membrane interaction. Thus, in contrast to the myristoylated alpha subunits which are tethered through their amino terminus, the carboxy-terminal residues of Gs alpha are required for association of this protein with the membrane.     

Influenza virus hemagglutinin containing an altered hydrophobic carboxy terminus accumulates intracellularly.

The influenza virus hemagglutinin (HA) glycoprotein synthesized from cloned DNA in a simian virus 40 vector is expressed on the surface of infected primate cells. Previously, it has been demonstrated that mutant HAs lacking the hydrophobic carboxy terminus fail to anchor on the cell surface and therefore are secreted extracellularly. During analysis of additional HA deletion mutants derived from an HA-simian virus 40 recombinant, we found a mutant with an altered hydrophobic carboxy terminus that exhibited another phenotype. This deletion mutant, dl-12, produced HA that was neither secreted nor expressed on the infected cell surface. The mutant HA was similar to the wild-type HA in apparent molecular weight and extent of glycosylation as assayed by endoglycosidase H sensitivity. The mutant HA localized near the perinuclear region of infected cells as indicated by an indirect immunofluorescence assay. Sequence analysis showed that a 5-base-pair deletion had occurred before the region encoding the hydrophobic carboxy terminus. Nevertheless, the physicochemical properties of the wild-type HA carboxy terminus were maintained in that the truncated HA carboxy terminus consisted of predominantly hydrophobic amino acids followed by several charged amino acids residues. This similarity in the carboxy terminus between the wild-type and mutant HAs may be responsible for the functional similarities observed. In spite of these similarities, the mutant HA failed to mature at the surface. These results suggest that the maturation of the mutant HA is blocked during a late stage in the transit to the cell surface.     

Phosphorylated Carboxy Terminal Serine Residues Stabilize the Mouse Gap Junction Protein Connexin45 Against Degradation

Phosphoamino acid analysis of mouse connexin45 (Cx45) expressed in human HeLa cells revealed that phosphorylation occurred mainly at serine residues, but also on tyrosine and threonine residues. To characterize the role of Cx45 phosphorylation, different serine residues of the serine-rich carboxy terminal region were deleted or exchanged for other amino acids residues. Human HeLa cells deficient in gap junctional intercellular communication were stably transfected with appropriate constructs and analyzed for expression, localization, phosphorylation, formation of functional gap junction channels and degradation of mutant Cx45. After exchange or deletion of nine carboxy terminal serine residues, phosphorylation was decreased by 90%, indicating that these serine residues represented main phosphorylation sites of mouse Cx45. The various serine residues of this region contributed differently to the phosphorylation of Cx45 suggesting a cooperative mechanism for phosphorylation. Substitution of different serine residues for other amino acids did not interfere with correct intracellular trafficking and assembly of functional gap junction channels, as shown by localization of mutant Cx45 at the plasma membrane and by dye transfer to neighboring cells. Truncated Cx45 was also weakly phosphorylated but was trapped in perinuclear locations. Dye transfer of these transfectants was similar as in nontransfected HeLa cells. The half-life of mouse Cx45 protein in HeLa cells was determined as 4.2 hr. Pulse-chase experiments with the different transfectants revealed an increased turnover of Cx45, when one or both of the serine residues at positions 381 and 382 or 384 and 385 were exchanged for other amino acids. The half-life of these mutants was diminished by 50% compared to wild type Cx45. Received: 26 September 1997/Revised: 5 January 1997     

Localization of the exposed N-terminal region of the B800-850 alpha and beta light-harvesting polypeptides on the cytoplasmic surface of Rhodopseudomonas capsulata chromatophores.

Proteinase K and trypsin were used to determine the orientation of the light-harvesting B800-850 alpha and beta polypeptides within the chromatophores (inside-out membrane vesicles) of the mutant strain Y5 of Rhodopseudomonas capsulata. With proteinase K 7 amino acid residues of the B800-850 alpha polypeptide were cleaved off up to position Trp-7--Thr-8 of the N terminus, and 11 residues were cleaved off up to position Leu-11-Ser-12 of the beta chain N terminus. The C termini of the B800-850 alpha and beta polypeptides, including the hydrophobic transmembrane portions, remained intact. It is proposed that the N termini of the alpha and beta subunits, each containing one transmembrane alpha-helical span, are exposed on the cytoplasmic membrane surface and the C termini are exposed to or directed toward the periplasm.     

Polyglutamine aggregation behavior in vitro supports a recruitment mechanism of cytotoxicity

The L1 major capsid proteins of human papillomavirus (HPV) types 11 and 16 were purified and analyzed for structural integrity and in vitro self-assembly. Proteins were expressed in Escherichia coli as glutathione-S-transferase-L1 (GST-L1) fusions and purified to near homogeneity as pentamers (equivalent to viral capsomeres), after thrombin cleavage from the GST moiety and removal of tightly associated GroEL protein. Sequences at the amino and carboxy termini contributing to formation of L1 pentamers and to in vitro capsid assembly were identified by deletion analysis. For both HPV11 and HPV16 L1, up to at least ten residues could be deleted from the amino terminus (Delta N10) and 30 residues from the carboxy terminus (Delta C30) without affecting pentamer formation. The HPV16 pentamers assembled into relatively regular, 72-pentamer shells ("virus-like particles" or VLPs) at low pH, with the exception of HPV16 L1 Delta N10, which assembled into a 12-pentamer, T=1 capsid (small VLP) under all conditions tested. The production of large quantities of assembly-competent L1, using the expression and purification protocol described here, has been useful for crystallographic analysis, and will be valuable for studies of virus-receptor interactions and potentially for vaccine design.     

The extracellular transport signal of the Vibrio cholerae endochitinase (ChiA) is a structural motif located between amino acids 75 and 555

ChiA, an 88-kDa endochitinase encoded by the chiA gene of the gram-negative enteropathogen Vibrio cholerae, is secreted via the eps-encoded main terminal branch of the general secretory pathway (GSP), a mechanism which also transports cholera toxin. To localize the extracellular transport signal of ChiA that initiates transport of the protein through the GSP, a chimera comprised of ChiA fused at the N terminus with the maltose-binding protein (MalE) of Escherichia coli and fused at the C terminus with a 13-amino-acid epitope tag (E-tag) was expressed in strain 569B(chiA::Kan(r)), a chiA-deficient but secretion-competent mutant of V. cholerae. Fractionation studies revealed that blockage of the natural N terminus and C terminus of ChiA did not prevent secretion of the MalE-ChiA-E-tag chimera. To locate the amino acid sequences which encoded the transport signal, a series of truncations of ChiA were engineered. Secretion of the mutant polypeptides was curtailed only when ChiA was deleted from the N terminus beyond amino acid position 75 or from the C terminus beyond amino acid 555. A mutant ChiA comprised of only those amino acids was secreted by wild-type V. cholerae but not by an epsD mutant, establishing that amino acids 75 to 555 independently harbored sufficient structural information to promote secretion by the GSP of V. cholerae. Cys77 and Cys537, two cysteines located just within the termini of ChiA(75-555), were not required for secretion, indicating that those residues were not essential for maintaining the functional activity of the ChiA extracellular transport signal.     

Characterization of a functional domain of human calpastatin

Expression plasmids were constructed from the cDNA of human calpastatin to examine the contribution to the inhibition of calpain of highly conserved sequences in each of four repetitive domains. A series of deletion derivatives of domain 1 proteins, truncated at either the amino or carboxy terminus, were produced in E. coli. Deletion from the amino terminus past the amino terminal conserved sequence decreased the inhibition. When the middle conserved sequence, the M-sequence, was further deleted, no inhibition was detected, but deletion from the carboxy terminus past the carboxy terminal conserved sequence did not decrease the inhibition until the M-sequence was reached. Nuclear magnetic resonance and circular dichroism spectra showed that domain 1 has an unfolded structure. Peptides that contained the M-sequence and some neighboring sequences were synthesized to measure the minimum size of the inhibitory peptide, which was the M-sequence with the next six residues on the amino terminal side.     

Expression and function of transplantation antigens with altered or deleted cytoplasmic domains

Two mutants of the class I gene encoding the H-2L^d transplantation antigen have been constructed. In one mutant the cytoplasmic domain of the class I molecule has been altered by deletion of 24 of the 31 C-terminal residues, and in the second the C-terminal 25 residues of the cytoplasmic domain have been replaced with a unique sequence of 19 amino acids. These mutant class I genes have been transferred into mouse L cells by DNA-mediated gene transfer. Both mutant genes are expressed at normal levels on the cell surface, and they have charge properties and sizes consistent with the introduced alterations. These mutant L^d molecules can serve as target antigens for allogeneic cytotoxic T cells and as restricting elements for virus-specific cytotoxic T cells. These results show that the 24 residues replaced or deleted from the carboxy terminus of the class I molecule are not required for its transport to or integration in the plasma membrane, nor for its function as a target antigen or a restricting element during T-cell-mediated cytotoxicity.     

Modification and processing of Bacillus licheniformis prepenicillinase in Escherichia coli. Fate of mutant penicillinase lacking lipoprotein modification site

We have previously shown that Bacillus licheniformis prepenicillinase is modified and processed to form membrane-bound penicillinase in Escherichia coli which contains N-acylglyceride-cysteine27 at the NH2 terminus. In the present study, we have constructed, by in vitro site-directed mutagenesis, two mutant penicillinase genes in which the modification site (the 27th cysteine residue in prepenicillinase) is either converted into serine (penPSer27) or is deleted along with the preceding four residues (Ala23 to Cys27, delta penP2327). The modification, processing, and subcellular localization of these two mutant penicillinases in E. coli cells were studied. Our results indicate that the delta penP2327 deletion mutant prepenicillinase is largely metabolically inert and the unmodified and uncleaved form is associated with the membrane fraction; a small fraction (about 7-9%) appears to contain glyceride-modified prepenicillinase (presumably at the Cys-21 position) which is not cleaved. In contrast, the Cys-27 in equilibrium Ser-27 point mutant prepenicillinase is processed into two forms which contain Asn-29 and Ser-35 at their NH2 termini, respectively, and the bulk of the processed penicillinase appears to be located in the peri-plasm. These results are discussed in terms of the substrate specificities of signal peptidases in E. coli.     

The 20 C-terminal amino acid residues of the chloroplast ATP synthase gamma subunit are not essential for activity.

It has been suggested that the last seven to nine amino acid residues at the C terminus of the gamma subunit of the ATP synthase act as a spindle for rotation of the gamma subunit with respect to the alpha beta subunits during catalysis (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). To test this hypothesis we selectively deleted C-terminal residues from the chloroplast gamma subunit, two at a time starting at the sixth residue from the end and finishing at the 20th residue from the end. The mutant gamma genes were overexpressed in Escherichia coli and assembled with a native alpha3beta3 complex. All the mutant forms of gamma assembled as effectively as the wild-type gamma. Deletion of the terminal 6 residues of gamma resulted in a significant increase (>50%) in the Ca-dependent ATPase activity when compared with the wild-type assembly. The increased activity persisted even after deletion of the C-terminal 14 residues, well beyond the seven residues proposed to form the spindle. Further deletions resulted in a decreased activity to approximately 19% of that of the wild-type enzyme after deleting all 20 C-terminal residues. The results indicate that the tip of the gammaC terminus is not essential for catalysis and raise questions about the role of the C terminus as a spindle for rotation.     

Pertussis toxin-catalyzed ADP-ribosylation of G(o) alpha with mutations at the carboxyl terminus.

The guanine nucleotide-binding protein G(o alpha) has been implicated in the regulation of Ca2+ channels in neural tissues. Covalent modification of G(o alpha) by pertussis toxin-catalyzed ADP-ribosylation of a cysteine (position 351) four amino acids from the carboxyl terminus decouples G(o alpha) from receptor. To define the structural requirements for ADP-ribosylation, preparations of recombinant G(o alpha) with mutations within the five amino acids at the carboxyl terminus were evaluated for their ability to serve as pertussis toxin substrates. As expected, the mutant in which cysteine 351 was replaced by glycine (C351G) was not a toxin substrate. Other inactive mutants were G352D and L353 delta/Y354 delta. Mutations that had no significant effect on toxin-catalyzed ADP-ribosylation included G350D, G350R, Y354 delta, and L353V/Y354 delta. Less active mutants were L353G/Y354 delta, L353A/Y354 delta, and L353G. ADP-ribosylation of the active mutants, like that of wild-type G(o alpha), was enhanced by the beta gamma subunits of bovine transducin. It appears that three of the four terminal amino acids critically influence pertussis toxin-catalyzed ADP-ribosylation of G(o alpha).     

The large cytoplasmic loop of the glucose transporter GLUT1 is an essential structural element for function.

Alanine scanning mutagenesis and the introduction of deletions and insertions were used to address the role of the large cytoplasmic loop in 2-deoxy-D-glucose (2-DOG) uptake by GLUT1 expressed in Xenopus oocytes. Alanine scanning mutagenesis of 29 amino acid residues that are identical or homologous in GLUT1 to GLUT4 demonstrated that the transport activities of only a few variants were affected. Progressive truncation of the loop by six deletions leaving intact 59 (delta236-241), 49 (delta231-246), 39 (delta226-251), 28 (delta221-257), 18 (delta216-262), or 10 (delta213-267) amino acid residues resulted in a progressive decrease in 2-DOG uptake. Compared with wild-type GLUT1 the uptake rates varied between 33% for the delta236-241 mutant and 4% for the delta213-267 mutant. Insertional mutagenesis using hexaalanine or hexaglycine to fill in the deletion 236D-241L restored 2-DOG uptake to 73% of wild-type GLUT1 in the case of hexaalanine, whereas hexaglycine insertion was without effect. Confocal laser microscopy demonstrated that a deletion of six amino acid residues did not influence the expression level in the plasma membrane (delta236-241 mutant), whereas the plasma membrane fluorescence of the delta213-267 mutant was comparable with that of water-injected Xenopus oocytes. Computer-aided secondary structure prediction of the loop suggested that it consists of a long alpha-helix bundle interrupted or kinked by the highly conserved glycine-233.     

Identification by molecular cloning of two forms of the alpha-subunit of the human liver stimulatory (GS) regulatory component of adenylyl cyclase

Two DNA molecules complementary to human liver mRNA coding for the alpha-subunit of the stimulatory regulatory component Gs of adenylyl cyclase were cloned. One of the two forms is a full-length cDNA of 1614 nucleotides plus a poly(A) tail of 59 nucleotides. The deduced sequence of 394 amino acids encoded by its open reading frame is essentially identical to that of the alpha-subunits of Gs identified by molecular cloning from bovine adrenals, bovine brain and rat brain. Two independent clones of the other type of cDNA were isolated. Both were incomplete, beginning within the open reading frame coding for the alpha s polypeptide. One codes for amino acids 5 through 394 and the other for amino acids 48 through 394 of the above described cDNA of 1614 nucleotides, and both have the identical 3'-untranslated sequence. They differ from the first cDNA, however, in that they lack a stretch of 42 nucleotides (numbers 214 through 255) and have nucleotides 213 (G) and 256 (G) replaced with C and A, respectively. This results in a predicted amino acid composition of another alpha-subunit of Gs that is shorter by 14 amino acids and contains two substitutions (Asp for Glu and Ser for Gly) at the interface between the deletion and the unchanged sequence. We call the smaller subunit alpha s1 and the larger alpha s2. This is the first demonstration of a structural heterogeneity in alpha s subunits that is due to a difference in amino acid sequence.     

Identification of a discrete intermediate in the assembly/disassembly of physalis mottle tymovirus through mutational analysis.

Assembly intermediates of icosahedral viruses are usually transient and are difficult to identify. In the present investigation, site-specific and deletion mutants of the coat protein gene of physalis mottle tymovirus (PhMV) were used to delineate the role of specific amino acid residues in the assembly of the virus and to identify intermediates in this process. N-terminal 30, 34, 35 and 39 amino acid deletion and single C-terminal (N188) deletion mutant proteins of PhMV were expressed in Escherichia coli. Site-specific mutants H69A, C75A, W96A, D144N, D144N-T151A, K143E and N188A were also constructed and expressed. The mutant protein lacking 30 amino acid residues from the N terminus self-assembled to T=3 particles in vivo while deletions of 34, 35 and 39 amino acid residues resulted in the mutant proteins that were insoluble. Interestingly, the coat protein (pR PhCP) expressed using pRSET B vector with an additional 41 amino acid residues at the N terminus also assembled into T=3 particles that were more compact and had a smaller diameter. These results demonstrate that the amino-terminal segment is flexible and either the deletion or addition of amino acid residues at the N terminus does not affect T=3 capsid assembly. In contrast, the deletion of even a single residue from the C terminus (PhN188Delta1) resulted in capsids that were unstable. These capsids disassembled to a discrete intermediate with a sedimentation coefficent of 19.4 S. However, the replacement of C-terminal asparagine 188 by alanine led to the formation of stable capsids. The C75A and D144N mutant proteins also assembled into capsids that were as stable as the pR PhCP, suggesting that C75 and D144 are not crucial for the T=3 capsid assembly. pR PhW96A and pR PhD144N-T151A mutant proteins failed to form capsids and were present as heterogeneous aggregates. Interestingly, the pR PhK143E mutant protein behaved in a manner similar to the C-terminal deletion protein in forming unstable capsids. The intermediate with an s value of 19.4 S was the major assembly product of pR PhH69A mutant protein and could correspond to a 30mer. It is possible that the assembly or disassembly is arrested at a similar stage in pR PhN188Delta1, pR PhH69A and pR PhK143E mutant proteins.     

Carboxy-terminal deletion analysis of oat phytochrome A reveals the presence of separate domains required for structure and biological activity.

A series of seven carboxy-terminal deletion mutants of oat phytochrome A were stably expressed in transgenic tobacco to localize phytochrome domains involved in chromophore attachment, spectral integrity, photoreversibility between the red light (Pr)- and far-red light (Pfr)-absorbing forms, dimerization, and biological activity. Amino acids necessary for chromophore attachment in vivo were localized to the amino-terminal 398 residues because mutant proteins this small had covalently bound chromophore. Deletion mutants from the carboxy terminus to residue 653 were spectrally indistinguishable from the full-length chromoprotein. In contrast, further truncation to residue 399 resulted in a chromoprotein with a bleached Pfr absorbance spectrum, Pr and Pfr absorbance maxima shifted toward shorter wavelengths, and reduced Pfr to Pr phototransformation efficiency. Thus, residues between 399 ad 652 are required for spectral integrity but are not essential for chromophore attachment. The sequence(s) between residues 919 and 1093 appears to be necessary for dimerization. Carboxy-terminal mutants containing this region behaved as dimers under nondenaturing conditions in vitro, whereas truncations without this region behaved as monomers. None of the plants expressing high levels of deletion mutants lacking the 35 carboxy-terminal amino acids displayed the light-exaggerated phenotype characteristic of plants expressing biologically active phytochrome A, even when the truncated phytochromes were expressed at levels 6- to 15-fold greater than that effective for the full-length chromoprotein. Collectively, these data show that the phytochrome protein contains several separable carboxy-terminal domains required for structure/function and identify a domain within 35 residues of the carboxy terminus that is critical for the biological activity of the photoreceptor in vivo.     

Studies on the regulatory domain of Ca2+/calmodulin-dependent protein kinase II by expression of mutated cDNAs in Escherichia coli.

The cDNAs encoding the alpha and beta subunits of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) were ligated into the bacterial expression vector pET and expressed in Escherichia coli. The bacterially expressed alpha and beta subunits exhibited Ca2+/calmodulin-dependent activity and were easily purified to apparent homogeneity from cell extracts. To determine the minimum size required for catalytic activity and the properties of the calmodulin-binding domain, mutated CaM kinase II cDNAs were expressed in E. coli and the enzymatic property of expressed proteins was examined. The replacement of Thr-286 of the alpha subunit with the negatively charged amino acid Asp or that of Arg-283 with the neutral amino acid Gly induced the partially Ca2+ independent activity. The mutant enzymes alpha-I(delta 283-478) and alpha-II(delta 359-478), which truncated the C-terminal region of the alpha subunit, exhibited CaM kinase II activity and the activities of alpha-I(delta 283-478) and alpha-II(delta 359-478) were completely independent of and partially dependent on Ca2+ and calmodulin, respectively. However, the truncated protein alpha(delta 250-478), which was only 33 amino acids shorter than the alpha-I(delta 283-478) protein had no enzymatic activity, indicating that alpha-I(delta 283-478) was close to the minimum size of the active form. The mutant enzyme alpha(delta 291-315), which lacked the calmodulin-binding domain exhibited Ca2+ independent activity. The molecular mass was, however, smaller than that expected from the amino acid sequence. The mutant enzyme alpha(delta 304-315), which lacked the C-terminal half of the calmodulin-binding domain of the alpha subunit, however, exhibited Ca(2+)-independent activity without a reduction in molecular size, indicating that residues 304-315 of the alpha subunit constituted the core calmodulin-binding domain.