Folding in vitro and transport in vivo of pre-β-lactamase are SecB independent

The rate of folding of the precursor of β-lactamase is not influenced by the presence of SecB under conditions in which GroEL/ES retards the folding. Wild-type β-lactamase and several mutants in the signal or the mature protein, affecting either transport or enzyme kinetics and probably folding, were examined for total expression, total enzymatic activity, and transported β-lactamase (in vivo resistance) in secB^- and secB⁺ strains. We conclude that there is no indication of any relevant interaction between SecB and pre-β-lactamase in vitro, nor did the secB^- mutation affect the transport of wild-type β-lactamase or any of the mutants in vivo. Thus, putative Escherichia coli'folding modulators'must be of limited specificity.

Role of a Candida albicans P1-type ATPase in resistance to copper and silver ion toxicity

Copper ion homeostasis is complicated in that copper is an essential element needed for a variety of cellular processes but is toxic at excess levels. To identify Candida albicans genes that are involved in resistance to copper ion toxicity, a library containing inserts of C. albicans genomic DNA was used to complement the copper sensitivity phenotype of a Saccharomyces cerevisiae cup1Delta strain that is unable to produce Cup1p, a metallothionein (MT) responsible for high-level copper ion resistance. A P1-type ATPase (CPx type) that is closely related to the human Menkes and Wilson disease proteins was cloned. The gene encoding this pump was termed CRD1 (for copper resistance determinant). A gene encoding a 76-amino-acid MT similar to higher eukaryotic MTs in structure was also cloned, and the gene was termed CRD2. Transcription of the CRD1 gene was found to increase upon growth with increasing copper levels, while the CRD2 mRNA was expressed at a constant level. Strains with the CRD1 gene disrupted were extremely sensitive to exogenous copper and failed to grow in medium containing 100 microM CuSO(4). These crd1 strains also exhibited increased sensitivity to silver and cadmium, indicating that Crd1p is somewhat promiscuous with respect to metal ion transport. Although strains with the CRD2 gene disrupted showed reduced growth rate with increasing copper concentration, the crd2 mutants eventually attained wild-type levels of growth, demonstrating that CRD2 is less important for resistance to copper ion toxicity. Crd1p is the first example of a eukaryotic copper pump that provides the primary source of cellular copper resistance, and its ability to confer silver resistance may enhance the prevalence of C. albicans as a nosocomial pathogen.

Complexes between protein export chaperone SecB and SecA. Evidence for separate sites on SecA providing binding energy and regulatory interactions

During localization to the periplasmic space or to the outer membrane of Escherichia coli some proteins are dependent on binding to the cytosolic chaperone SecB, which in turn is targeted to the membrane by specific interaction with SecA, a peripheral component of the translocase. Five variant forms of SecB, previously demonstrated to be defective in mediating export in vivo (Gannon, P. M., and Kumamoto, C. A. (1993) J. Biol. Chem. 268, 1590-1595; Kimsey, H. K., Dagarag, M. D., and Kumamoto, C. A. (1995) J. Biol. Chem. 270, 22831-22835) were investigated with respect to their ability to bind SecA both in solution and at the membrane translocase. We present evidence that at least two regions of SecA are involved in the formation of active complexes with SecB. The variant forms of SecB were all capable of interacting with SecA in solution to form complexes with stability similar to that of complexes between SecA and wild-type SecB. However, the variant forms were defective in interaction with a separate region of SecA, which was shown to trigger a change that was correlated to activation of the complex. The region of SecA involved in activation of the complexes was defined as the extreme carboxyl-terminal 21 aminoacyl residues.

Intestinal lesions associated with disseminated candidiasis in an experimental animal model

In human patients, disseminated candidiasis, a life-threatening disease for immunocompromised patients, is often associated with intestinal lesions. In this study, we demonstrate that immunosuppressed gnotobiotic (IGB) piglets orally inoculated with wild-type Candida albicans developed extensive intestinal lesions and disseminated infection. Severe ulceration of the ileal mucosa was observed overlying regions of colonization and necrosis of the gut-associated lymphoid tissue. Despite the high susceptibility of IGB piglets to many microbial pathogens, an avirulent mutant strain of C. albicans failed to produce intestinal lesions and exhibited poor dissemination, demonstrating that these effects required virulent organisms. It is likely that in IGB piglets, as in human patients, intestinal lesions provide the mechanism for escape of C. albicans from the gastrointestinal tract. Multinucleated giant cells containing fungal organisms were observed within lymph nodes and lymphatic vessels, and as with other pathogens, such cells could provide a mechanism for dissemination of C. albicans.

A highly mobile C-terminal tail of the Escherichia coli protein export chaperone SecB

The Escherichia coli export chaperone SecB binds nascent precursors of certain periplasmic and outer membrane proteins and prevents them from folding or aggregating in the cytoplasm. In this study, we demonstrate that the C-terminal 13 residues of SecB were highly mobile using (1)H NMR spectroscopy. A protein lacking the C-terminal 13 amino acids of wild-type SecB was found to retain the ability to bind unfolded maltose-binding protein (MBP) in vitro but to interfere with the normal kinetics of pre-MBP export when overexpressed in vivo. The defect in export was reversed by overproduction of the peripheral membrane ATPase SecA. Therefore, deletion of the mobile region of SecB may alter the interactions of SecB with SecA.

Filamentous growth of Candida albicans in response to physical environmental cues and its regulation by the unique CZF1 gene

Hyphal growth in the opportunistic fungal pathogen Candida albicans is believed to contribute to the virulence of the organism by promoting penetration of fungal cells into host tissue. In this study, stimulation of hyphal growth by a feature of the physical environment was demonstrated. Specifically, growth of cells embedded within a matrix promoted the formation of hyphae. The CZF1 gene, encoding a putative transcription factor, was shown to be involved in the regulation of hyphal growth under certain conditions, including embedded conditions. Ectopic expression of CZF1 in embedded cells promoted the rapid formation of hyphae. Elimination of CZF1 and CPH1, encoding a homologue of the Saccharomyces cerevisiae Ste12p transcription factor, led to a pronounced defect in filamentous growth of embedded cells. Elimination of CZF1 alone led to a moderate defect in hyphal growth under some conditions, including embedded conditions. Hyphal morphogenesis in response to matrix embedding may occur in the opportunistic pathogen, C. albicans, to promote invasion of fungal cells into host tissue.

Mutational alterations in the homotetrameric chaperone SecB that implicate the structure as dimer of dimers

Variant forms of SecB with substitutions of aminoacyl residues in the region from 74 to 80 were analyzed with respect to their ability to bind a physiological ligand, precursor galactose-binding protein, and to their oligomeric states. SecBL75Q and SecBE77K are tetramers with affinity for ligand indistinguishable from that of the wild-type SecB, and thus the export defect exhibited by strains producing these variants must result from an effect on interactions between SecB and other components. SecBF74I is tetrameric but binds ligand with a lower affinity. Substitutions at positions 76, 78, and 80 cause a shift in the equilibrium so that the SecB tetramer dissociates into dimers. We conclude that the tetramer is a dimer of dimers and that the residues Cys76, Val78, and Gln80 must be involved either directly or indirectly in forming the interface between dimers. These variant species are defective in binding ligand; however, because their oligomeric state is altered no conclusion can be drawn concerning the direct role of these residues in ligand binding.

Invasive lesions containing filamentous forms produced by a Candida albicans mutant that is defective in filamentous growth in culture

A Candida albicans efg1 cph1 double mutant is nonfilamentous under standard laboratory conditions and avirulent in mice. However, this mutant produced filaments in the tongues of immunosuppressed gnotobiotic piglets and when embedded in agar, demonstrating that an Efg1p- and Cph1p-independent pathway for promotion of filamentous growth exists.

Overproduction of SecA suppresses the export defect caused by a mutation in the gene encoding the Escherichia coli export chaperone secB

SecB is a cytosolic protein required for rapid and efficient export of particular periplasmic and outer membrane proteins in Escherichia coli. SecB promotes export by stabilizing newly synthesized precursor proteins in a nonnative conformation and by targeting the precursors to the inner membrane. Biochemical studies suggest that SecB facilitates precursor targeting by binding to the SecA protein, a component of the membrane-embedded translocation apparatus. To gain more insight into the functional interaction of SecB and SecA, in vivo, mutations in the secA locus that compensate for the export defect caused by the secB missense mutation secBL75Q were isolated. Two suppressors were isolated, both of which led to the overproduction of wild-type SecA protein. In vivo studies demonstrated that the SecBL75Q mutant protein releases precursor proteins at a lower rate than does wild-type SecB. Increasing the level of SecA protein in the cell was found to reverse this slow-release defect, indicating that overproduction of SecA stimulates the turnover of SecBL75Q-precursor complexes. These findings lend additional support to the proposed pathway for precursor targeting in which SecB promotes targeting to the translocation apparatus by binding to the SecA protein.

Preprotein transfer to the Escherichia coli translocase requires the co-operative binding of SecB and the signal sequence to SecA

In Escherichia coli, precursor proteins are targeted to the membrane-bound translocase by the cytosolic chaperone SecB. SecB binds to the extreme carboxy-terminus of the SecA ATPase translocase subunit, and this interaction is promoted by preproteins. The mutant SecB proteins, L75Q and E77K, which interfere with preprotein translocation in vivo, are unable to stimulate in vitro translocation. Both mutants bind proOmpA but fail to support the SecA-dependent membrane binding of proOmpA because of a marked reduction in their binding affinities for SecA. The stimulatory effect of preproteins on the interaction between SecB and SecA exclusively involves the signal sequence domain of the preprotein, as it can be mimicked by a synthetic signal peptide and is not observed with a mutant preprotein (delta8proOmpA) bearing a non-functional signal sequence. Delta8proOmpA is not translocated across wild-type membranes, but the translocation defect is suppressed in inner membrane vesicles derived from a prIA4 strain. SecB reduces the translocation of delta8proOmpA into these vesicles and almost completely prevents translocation when, in addition, the SecB binding site on SecA is removed. These data demonstrate that efficient targeting of preproteins by SecB requires both a functional signal sequence and a SecB binding domain on SecA. It is concluded that the SecB-SecA interaction is needed to dissociate the mature preprotein domain from SecB and that binding of the signal sequence domain to SecA is required to ensure efficient transfer of the preprotein to the translocase.

Genetic analysis in fungi using restriction-enzyme-mediated integration

Restriction-enzyme-mediated integration (REMI), a method for generating nonhomologous integration of transforming DNA into the chromosomes of eukaryotic cells, has been used for insertion mutagenesis and other genetic studies in diverse organisms. Insertion mutations generated by REMI have facilitated the genetic dissection of developmental pathways in Dictyostelium discoidium and the isolation of virulence factors in several plant pathogenic fungi. Recent work indicates that REMI occurs by nonhomologous end joining.

Escherichia coli SecB stimulates export without maintaining export competence of ribose-binding protein signal sequence mutants

Ribose-binding protein (RBP) is exported to the periplasm of Escherichia coli via the general export pathway. An rbsB-lacZ gene fusion was constructed and used to select mutants defective in RBP export. The spontaneous Lac+ mutants isolated in this selection contained either single-amino-acid substitutions or a deletion of the RBP signal sequence. Intact rbsB genes containing eight different point mutations in the signal sequence were reconstructed, and the effects of the mutations on RBP export were examined. Most of the mutations caused severe defects in RBP export. In addition, different suppressor mutations in SecY/PrlA protein were analyzed for their effects on the export of RBP signal sequence mutants in the presence or absence of SecB. Several RBP signal sequence mutants were efficiently suppressed, but others were not suppressed. Export of an RBP signal sequence mutant in prlA mutant strains was partially dependent on SecB, which is in contrast to the SecB independence of wild-type RBP export. However, the kinetics of export of an RBP signal sequence mutant point to a rapid loss of pre-RBP export competence, which occurs in strains containing or lacking SecB. These results suggest that SecB does not stabilize the export-competent conformation of RBP and may affect translocation by stabilizing the binding of pre-RBP at the translocation site.

Stable transformation and regulated expression of an inducible reporter construct in Candida albicans using restriction enzyme-mediated integration

To allow the regulated expression of cloned genes in Candida albicans, a plasmid was constructed using the inducible promoter of the C. Albicans MAL2 gene. To demonstrate that the MAL2 promoter could regulate cloned genes placed under its control, a fusion construct was made with the coding sequence of the C. albicans URA3 gene. This plasmid was introduced into a Ura- strain of C. albicans using the process of restriction enzyme-mediated integration (REMI). This procedure involves the transformation of the BamHI-linearized plasmid in the presence of BamHI enzyme. The majority of transformants generated contained insertions of the plasmid at chromosomal BamHI sites. All transformants examined were inducible for URA3 expression, which was determined by growth analysis and by measuring the level of URA3 gene product activity. The URA+ phenotype of the transformants was stable during growth under nonselective conditions. This system offers the advantages of stable transformation, easy recovery of integrated DNA, and inducible expression of genes in C. albicans.

Diverse effects of mutation on the activity of the Escherichia coli export chaperone SecB

The Escherichia coli SecB protein binds newly synthesized precursor maltose-binding protein (preMBP) and promotes its rapid export from the cytoplasm. Site-directed mutagenesis of two regions of SecB was carried out to better understand factors governing the SecB.preMBP interaction. 30 aminoacyl substitution mutants were analyzed, revealing two distinct classes of secB mutants. Substitutions at the alternating positions Phe-74, Cys-76, Val-78, or Gln-80 reduced the ability of SecB to form stable complexes with preMBP, but caused only mild defects in the rate of MBP export from living cells. The pattern revealed by this class of mutants suggests that a primary binding site for preMBP is hydrophobic and contains beta-sheet secondary structure. In contrast, substitutions at Asp-20, Glu-24, Leu-75, or Glu-77 caused a severe slowing in the rate of MBP export but did not disrupt SecB.preMBP complex formation. These largely acidic residues may function to regulate the opening of a preprotein binding site, allowing both high affinity preprotein binding and rapid dissociation of SecB.preprotein complexes at the membrane translocation site.

prlA suppression of defective export of maltose-binding protein in secB mutants of Escherichia coli

An Escherichia coli strain containing a signal sequence mutation in the periplasmic maltose-binding protein (MBP) (malE18-1) and a point mutation in the soluble export factor SecB (secBL75Q) is completely defective in export of MBP and unable to grow on maltose (Mal- phenotype). We isolated 95 spontaneous Mal+ revertants and characterized them genetically. Three types of extragenic suppressors were identified: informational (missense) suppressors, a bypass suppressor conferring the Mal+ phenotype in the absence of MBP, and suppressors affecting the prlA gene, which encodes a component of the protein export apparatus. In this study, a novel prlA allele, designated prlA1001 and mapping in the putative second transmembrane domain of the PrlA (SecY) protein, was found. In addition, we isolated a mutation designated prlA1024 which is identical to prlA4-2, the mutation responsible for the signal sequence suppression in the prlA4 (prlA4-1 prlA4-2) double mutant (T. Sako and T. Iino, J. Bacteriol. 170:5389-5391, 1988). Comparison of the prlA1024 mutant and the prlA4 double mutant provides a possible explanation for the isolation of these prlA alleles.

Highly selective binding of nascent polypeptides by an Escherichia coli chaperone protein in vivo

Chaperone proteins bind to newly synthesized polypeptides and assist in various assembly reactions. The Escherichia coli chaperone protein SecB binds precursors of exported proteins and assists in export. In vitro, SecB can bind to many unfolded proteins. In this report, we demonstrate that SecB binding in vivo is highly selective; the major polypeptides that are bound by SecB are nascent precursors of the exported proteins maltose-binding protein (MBP), LamB, OmpF, and OmpA. These results support the hypothesis that the primary physiological function of SecB is to stimulate protein export. By interacting with nascent polypeptides, SecB probably stimulates their cotranslational association with the membrane-bound protein translocation apparatus.

A mutation of Escherichia coli SecA protein that partially compensates for the absence of SecB

The Escherichia coli SecB protein is a cytosolic chaperone protein that is required for rapid export of a subset of exported proteins. To aid in elucidation of the activities of SecB that contribute to rapid export kinetics, mutations that partially suppressed the export defect caused by the absence of SecB were selected. One of these mutations improves protein export in the absence of SecB and is the result of a duplication of SecA coding sequences, leading to the synthesis of a large, in-frame fusion protein. Unexpectedly, this mutation conferred a second phenotype. The secA mutation exacerbated the defective protein export caused by point mutations in the signal sequence of pre-maltose-binding protein. One explanation for these results is that the mutant SecA protein has sustained a duplication of its binding site(s) for exported protein precursors so that the mutant SecA is altered in its interaction with precursor molecules.

Mutations of the molecular chaperone protein SecB which alter the interaction between SecB and maltose-binding protein

SecB is a 16-kDa cytosolic chaperone protein that is required for efficient export of particular proteins in Escherichia coli. To identify regions of SecB that contribute to efficient protein export, we isolated secB point mutants that are defective for protein export in vivo. We obtained missense mutations at residues Leu75 (SecBL75Q), Cys76 (SecBC76Y), and Glu77 (SecBE77K) in the center of the secB gene. In vivo, mutant SecBL75Q and SecBE77K proteins are capable of binding to precursor maltose-binding protein (MBP) and preventing the formation of export-incompetent precursor MBP; however, export of MBP is still defective. In vitro, purified SecBL75Q and SecBE77K proteins bound to unfolded MBP and blocked its refolding. SecBL75Q and SecBE77K were more effective than wild-type SecB at blocking the refolding of unfolded MBP, suggesting that SecBL75Q and SecBE77K have a higher affinity for unfolded MBP.

Heat-shock proteins can substitute for SecB function during protein export in Escherichia coli

In this study we have shown that (i) induction of the heat-shock response can substitute for SecB function in Escherichia coli, (ii) SecB itself is not a heat-shock protein and (iii) a basal level of heat-shock proteins is required for cells to grow in the absence of a functional SecB protein. Overproduction of DnaK, or GroEL/ES, which were candidates for the heat-shock proteins that could substitute for SecB function, did not rescue the export defect caused when SecB was limiting or absent. In an attempt to identify the heat-shock protein(s) which could substitute for SecB function, unlinked suppressors of secB were isolated and characterized. Interestingly, most of the suppressors mapped to the rpoH locus. Since rpoH encodes sigma 32, the heat-shock transcription factor, it is likely that these suppressors affect the synthesis levels of heat-shock proteins that can substitute for SecB function. The remaining suppressors did not map to any known heat-shock or export genes. Collectively, our data suggest that these suppressors may represent unidentified heat-shock proteins or export factors that act in a manner similar to SecB in facilitating the export process in E. coli.

Molecular chaperones and protein translocation across the Escherichia coli inner membrane

Proteins that are able to translocate across biological membranes assume a loosely folded structure. In this review it is suggested that the loosely folded structure, referred to here as the 'pre-folded conformation', is a particular structure that interacts favourably with components of the export apparatus. Two soluble factors, SecB and GroEL, have been implicated in maintenance of the pre-folded conformation and have been termed 'molecular chaperones'. Results suggest that SecB may be a chaperone that is specialized for binding to exported protein precursors, while GroEL may be a general folding modulator that binds to many intracellular proteins.

Folding in vitro and transport in vivo of pre-beta-lactamase are SecB independent

The rate of folding of the precursor of beta-lactamase is not influenced by the presence of SecB under conditions in which GroEL/ES retards the folding. Wild-type beta-lactamase and several mutants in the signal or the mature protein, affecting either transport or enzyme kinetics and probably folding, were examined for total expression, total enzymatic activity, and transported beta-lactamase (in vivo resistance) in secB- and secB+ strains. We conclude that there is no indication of any relevant interaction between SecB and pre-beta-lactamase in vitro, nor did the secB- mutation affect the transport of wild-type beta-lactamase or any of the mutant in vivo. Thus, putative Escherichia coli "folding modulators' must be of limited specificity.

The presence of both the signal sequence and a region of mature LamB protein is required for the interaction of LamB with the export factor SecB

In the accompanying paper (Altman, E., Bankaitis, V.A., and Emr, S.D. (1990) J. Biol. Chem. 265, 18148-18153) a putative SecB binding site was identified in the mature LamB protein. The export of wild-type LamB was unperturbed when this region was removed, however, suggesting the presence of a second site of interaction between SecB and LamB. In this paper we show that the interference caused by export-defective LamB proteins is influenced by the amount of signal sequence that is present. If a large portion of the signal sequence is deleted then the interference levels are significantly reduced. This result suggests that a region of the signal sequence contributes to the interaction of SecB with the LamB protein. Using anti-SecB affinity chromatography, we demonstrated directly that the association of SecB protein with precursor LamB is dependent on the presence of both the LamB signal sequence and the interfering region which maps to amino acids 320-380 of mature LamB. Although the interfering region is not necessary for the export of wild-type LamB under normal conditions, when the signal sequence is mutationally altered the interfering region is required to promote the efficient export of LamB protein. Also, deletion of the interfering region eliminates the ability of wild-type LamB precursor to be maintained in an export competent conformation in vivo. Collectively, our results indicate that efficient export of the LamB protein is achieved by an interaction with SecB that involves both the LamB signal sequence and the interfering region in mature LamB.

SecB protein: a cytosolic export factor that associates with nascent exported proteins

Soluble factors participate in protein translocation across a variety of biological membranes. The Escherichia coli soluble protein SecB (the product of the secB gene) is involved in the export of periplasmic and outer membrane proteins. The isolation of secB mutations permitted the demonstration that SecB is required for rapid and efficient export of certain proteins. Consistent with the results of these genetic studies, purified SecB has been shown to stimulate protein translocation across E. coli inner membrane vesicles in vitro. This article presents a review of these past studies of SecB, speculation on the role of SecB in protein translocation, and a comparison of SecB and other factors, trigger factor and GroEL.

Three pure chaperone proteins of Escherichia coli--SecB, trigger factor and GroEL--form soluble complexes with precursor proteins in vitro

Diverse studies of three cytoplasmic proteins of Escherichia coli--SecB, trigger factor and GroEL--have suggested that they can maintain precursor proteins in a conformation which is competent for membrane translocation. These proteins have been termed 'chaperones'. Using purified chaperone proteins and precursor protein substrates, we find that each of these chaperones can stabilize proOmpA for translocation and for the translocation-ATPase. These chaperones bind to proOmpA to form isolable complexes. SecB and GroEL will also form complexes with another exported protein, prePhoE. In contrast, these chaperones do not form stable complexes with a variety of soluble proteins such as SecA protein, bovine serum albumin, ovalbumin or ribonuclease A. While chaperones may transiently interact with soluble proteins to catalyze their folding, the stable interaction between chaperones and presecretory proteins, maintaining an open conformation which is essential for translocation, may commit these proteins to the secretion pathway.

Escherichia coli SecB protein associates with exported protein precursors in vivo

The product of the Escherichia coli secB gene is required for efficient export of proteins across the cytoplasmic membrane. The studies described in this report show that in wild-type growing cells, SecB protein associates with precursor forms of exported proteins, such as the periplasmic maltose-binding protein (MBP) and the outer-membrane proteins LamB and OmpA. In contrast, the cytoplasmic protein beta-galactosidase was not found in association with SecB. Pulse-chase analysis showed that the SecB-precursor MBP complex was short lived, as expected for a complex that represents an intermediate in the protein-export pathway. The results support the hypothesis that SecB protein associates with exported protein precursors in the cytoplasm and dissociates prior to or during translocation of precursors across the cell membrane.

Purification of the Escherichia coli secB gene product and demonstration of its activity in an in vitro protein translocation system

Mutations in the Escherichia coli secB gene lead to protein export defects in vivo (Kumamoto, C.A., and Beckwith, J. (1985) J. Bacteriol. 163, 267-274). To demonstrate directly the participation of the secB gene product (SecB) in protein export, SecB was purified, and its effects on in vitro protein translocation were analyzed. SecB was purified from soluble extracts of a strain that overproduced it, by ammonium sulfate precipitation, DEAE-cellulose chromatography, and differential precipitation at acid pH. The chromatographic behavior on gel filtration columns indicated apparent molecular masses of approximately 90 kDa for both purified SecB and SecB in cytosolic extracts of wild type cells. When added to a translocation mixture, purified SecB stimulated pro-OmpA translocation into membrane vesicles. SecB also suppressed the thermoinduced defect in translocating activity of membranes derived from a secY24 mutant. The results of these in vitro studies and of previous in vivo studies demonstrate that SecB plays a direct role in normal protein export in E. coli.

The mature portion of Escherichia coli maltose-binding protein (MBP) determines the dependence of MBP on SecB for export

The product of the secB gene is required for export of a subset of secreted proteins to the outer membrane and periplasm of Escherichia coli. Precursor maltose-binding protein (MBP) accumulates in the cytoplasm of secB-carrying mutants, but export of alkaline phosphatase is only minimally affected by secB mutations. When export of MBP-alkaline phosphatase hybrid proteins was analyzed in wild-type and secB-carrying mutant strains, the first third of mature MBP was sufficient to render export of the hybrid proteins dependent on SecB. Substitution of a signal sequence from a SecB-independent protein had no effect on SecB-dependent export. These findings show that the first third of mature MBP is capable of conferring export incompetence on an otherwise competent protein.

Characterization of the Escherichia coli protein-export gene secB

The Escherichia coli secB gene product is required for normal export of envelope proteins out of the cell cytoplasm. In this report, we present the identification and nucleotide sequence of the secB coding sequence. The secB structural gene overlaps almost completely with a predicted open reading frame (ORF) that is encoded on the opposite strand. To establish the identity of the secB ORF, we characterized a secB mutation that caused total loss of secB function, based upon its phenotype. This mutation resulted from a nucleotide change that caused an ochre mutation in one ORF (the secB gene) and a silent (no amino acid change) codon change in the opposite ORF.

Effects of Escherichia coli secB mutations on pre-maltose binding protein conformation and export kinetics

Mutations affecting the secB gene of Escherichia coli cause a defect in protein export. This report presents the demonstration that the secB mutations caused a defect in co-translational processing of maltose binding protein (MBP). A significant amount of post-translational processing of pre-MBP occurred within 1 min after termination of pulse labeling; at later time points only a small amount of additional processing occurred. The conformation of the intracellular precursor form of MBP was examined in a secB::Tn5 mutant, using protease sensitivity (Randall, L. L., and Hardy, S. J. S. (1986) Cell 46, 921-928) as the assay. In contrast to the isogenic wild type strain, a population of pre-MBP that had folded into a protease resistant conformation was detected in the secB mutant. In addition, sublethal doses of chloramphenicol did not significantly affect protein export in the secB::Tn5 mutant and the secB::Tn5 mutation did not lead to defects in membrane energization.

A mutation of the c subunit of the Escherichia coli proton-translocating ATPase that suppresses the effects of a mutant b subunit

A mutation of the b subunit of the Escherichia coli proton-translocating ATPase and mutations in the gene for the a subunit that suppress its effects have been previously described (Kumamoto, C., and Simoni, R. D. (1986) J. Biol. Chem. 261, 10037-10042). In this paper, we describe the characterization of a new mutation that partially suppresses the effects of the original b mutation. The new suppressor mutation causes the substitution of serine for alanine at position 62 of the c subunit. Biochemical studies of double mutants, carrying both b and c mutations, demonstrate that the c mutation partially restores the function of the enzyme complex.

Genetic evidence for interaction between the a and b subunits of the F0 portion of the Escherichia coli proton translocating ATPase

A mutation of the b subunit of the Escherichia coli proton translocating ATPase was previously described (Porter, A. C. G., Kumamoto, C., Aldape, K., and Simoni, R. D. (1985) J. Biol. Chem. 260, 8182-8187). This mutation, which causes substitution of aspartic acid for glycine at position 9 (basp9), results in loss of function of the ATPase complex. In this paper we describe the isolation and characterization of two mutations that partially suppress the effects of the basp9 alteration. The suppressor mutations cause amino acid substitutions at position 240 of the a subunit. Membranes derived from strains carrying a suppressor mutation and the basp9 mutation exhibited ATP-dependent proton translocating activity.

Does secA mediate coupling between secretion and translation in Escherichia coli?

An amber mutation in the secA gene of Escherichia coli causes a pleiotropic decrease in the synthesis of secreted proteins, including maltose-binding protein (MBP) and alkaline phosphatase. Reversal of the inhibition of MBP synthesis in secA(Am) strains by signal sequence mutations in the malE gene has been reported. These results suggest a coupling between secretion and translation which involves an interaction between the signal sequence of nascent polypeptides and a cellular secretion machinery. Further analysis reported here indicated that signal sequence mutations of MBP or alkaline phosphatase did not selectively overcome the inhibition of MBP or alkaline phosphatase synthesis in secA(Am) strains. Rather, at a given time in parallel experiments there was substantial variability among closely isogenic secA(Am) strains in the magnitude of the synthesis block; this variability could account for the earlier results. Further experiments suggested that the inhibition of MBP synthesis in secA(Am) strains was caused by depletion of cyclic AMP, leading to decreased transcription of the malE gene. However, the secretion defects in secA(Am) strains were not affected by cyclic AMP levels. Therefore, we conclude that the reduction in MBP synthesis was a secondary consequence of the primary export defect in the secA(Am) strains.

Evidence for specificity at an early step in protein export in Escherichia coli

We previously described mutations in a gene, secB, which have pleiotropic effects on protein export in Escherichia coli. In this paper, we report the isolation of mutants in which the activity of the secB gene was eliminated. Null mutations in secB affected only a subset of exported proteins. Strains carrying these mutations, although unable to grow on L broth plates, were still viable on minimal media. These secB mutations reversed a block in the translation of an exported protein that was caused by the elimination of another component of the secretion machinery, SecA protein. These results suggest that the secB product acts at an early step in the export process and is involved in the export of only a subset of cell envelope proteins.

Signal sequence mutations disrupt feedback between secretion of an exported protein and its synthesis in E. coli

Recent studies in a eukaryotic system indicate that a block in secretion can lead to a block in the translation of secretory proteins. This feedback on protein synthesis is thought to be a result of an interaction of the signal recognition particle with the signal sequences of nascent proteins. Genetic studies in the prokaryote Escherichia coli suggest that a complex secretion machinery and a similar feedback mechanism exist. In addition, mutations affecting two genes, secA and secC, thought to encode components of the bacterial secretion machinery, selectively interfere with the synthesis of exported proteins. This selective interference with translation may be a result of recognition by the secretion machinery of signal sequences. If so, alteration of the signal sequence of a particular protein by mutation should eliminate the block in synthesis for that protein. We show here that signal sequence mutants for an exported protein, maltose binding protein, prevent the block in synthesis of this protein in a secA mutant.

Mutations in a new gene, secB, cause defective protein localization in Escherichia coli

We isolated a new class of Escherichia coli mutants with pleiotropic defects in protein secretion. Using a previously described selection procedure (Oliver et al., Ann. Microbiol. [Paris] 133A:105-110, 1982), we obtained a large collection of strains containing mutations that affect protein localization. In many cases, the lesions causing the secretion defects were mapped in or near the previously identified gene, secA (Oliver and Beckwith, Cell 25:765-772, 1981). However, the selection also yielded mutants with mutations in a new locus, which was designated secB. These secB mutants were defective in the localization of maltose-binding protein and, in at least one case, OmpF protein. Double mutants with lesions in both secA and secB had strong defects in the secretion of maltose-binding protein and OmpF protein. The secB locus mapped near cysE at min 80.5 on the E. coli genetic map. The properties of secB mutants suggest that the secB product could be a component of the E. coli secretory apparatus.

Assembly and processing of procollagen V (AB) in chick blood vessels and other tissues

The biosynthesis and processing of type V procollagens was investigated in chick embryo blood vessels labeled with radioactive amino acids. Monomeric, pepsin-sensitive pro alpha 1 V and pro alpha 2 V chains are slowly assembled into triple helically folded molecules. A small proportion of these procollagen V molecules contain interchain disulfide bridges, and the disulfide-linked heterodimer and heterotrimer (pro alpha 1 V)2pro alpha 2 V were found. A relatively fast conversion of procollagen V to p-collagen V is followed by slow change to collagen V. This time course is similar to the processing of procollagen III and is much slower than the rate of appearance of type I collagen in blood vessels. A combination of sedimentation and electrophoretic analyses was used to measure the relative size of the type V chains and to demonstrate attachment of noncollagenous peptides (Mr = 33,000) to p alpha 2 V by disulfide linkage. Similar quantitative pulse-chase studies were made with calvaria and crop. As the same unusual features of assembly and processing of type V chains were seen in muscle, bone, and blood vessels, we conclude that these are characteristic of type V collagen biosynthesis.

Propeptides of procollagen V (A,B) in chick embryo crop

The biosynthesis of type V (A,B) collagens was recently found to proceed through the sequential forms pro alpha (A,B), p alpha (A,B), and f alpha (A,B). All these chains are larger than the A,B chains extracted from tissues after pepsin digestion. This report shows that all forms contain substantial peptides which are resistant to bacterial collagenase and concludes that type V collagens differ from the intestitial collagens (types I, II, III) in retaining large noncollagenous peptides in tissues. A peptide becomes transiently attached to the processing intermediate p alpha A by a reducible linkage. The conversion of procollagen V to p-collagen V was inhibited by colchicine and arginine. Previously, the disulfide-linked heterotrimer [(pro alpha B)2 (pro alpha A)] was found and additional procollagens containing only B type molecules were inferred. Further investigations reported here agree with these conclusions and also indicate that some trimeric molecules containing more than one chain related to A may exist.

Biosynthesis of A,B procollagen

Interest in cell-associated collagens led others to isolate A and B collagen chains, also known as type V collagen, from many tissues, but only after pepsin cleavage. Soluble precursors of these chains are synthesized in vitro by crop, a chicken embryo muscle tissue, and are converted by at least two processing steps from procollagens via intermediates to final forms which are large than the pepsin-derived A and B chains. Heterotrimeric, disulfide-bridged procollagen molecules corresponding to B2A exist. Components were separated by ion exchange chromatography, velocity sedimentation, and electrophoresis, and the relationships between them were established by sequential radioactive labeling and comparison of peptides generated by protease cleavage.