Characterization of the putative type III secretion ATPase CdsN (Cpn0707) of Chlamydophila pneumoniae

Type III secretion (T3S) is utilized by a wide range of gram-negative bacterial pathogens to allow the efficient delivery of effector proteins into the host cell cytoplasm through the use of a syringe-like injectisome. Chlamydophila pneumoniae is a gram-negative, obligate intracellular pathogen that has the structural genes coding for a T3S system, but the functionality of the system has not yet been demonstrated. T3S is dependent on ATPase activity, which catalyzes the unfolding of proteins and the secretion of effector proteins through the injectisome. CdsN (Cpn0707) is predicted to be the T3S ATPase of C. pneumoniae based on sequence similarity to other T3S ATPases. Full-length CdsN and a C-terminal truncation of CdsN were cloned as glutathione S-transferase (GST)-tagged constructs and expressed in Escherichia coli. The GST-tagged C-terminal truncation of CdsN possessed ATPase activity, catalyzing the release of ADP and P(i) from ATP at a rate of 0.55 +/- 0.07 micromol min(-1) mg(-1) in a time- and dose-dependent manner. CdsN formed oligomers and high-molecular-weight multimers, as assessed by formaldehyde fixation and nondenaturing polyacrylamide gel electrophoresis. Using bacterial two-hybrid and GST pull-down assays, CdsN was shown to interact with CdsD, CdsL, CdsQ, and CopN, four putative structural components of the C. pneumoniae T3S system. CdsN also interacted with an unannotated protein, Cpn0706, a putative CdsN chaperone. Interactions between CdsN, CdsD, and CopN represent novel interactions not previously reported for other bacterial T3S systems and may be important in the localization and/or function of the ATPase at the inner membrane of C. pneumoniae.     

Interactions between CdsD, CdsQ, and CdsL, three putative Chlamydophila pneumoniae type III secretion proteins

Chlamydophila pneumoniae is a gram-negative obligate intracellular bacterial pathogen that causes pneumonia and bronchitis and may contribute to atherosclerosis. The developmental cycle of C. pneumoniae includes a morphological transition from an infectious extracellular elementary body (EB) to a noninfectious intracellular reticulate body (RB) that divides by binary fission. The C. pneumoniae genome encodes a type III secretion (T3S) apparatus that may be used to infect eukaryotic cells and to evade the host immune response. In the present study, Cpn0712 (CdsD), Cpn0704 (CdsQ), and Cpn0826 (CdsL), three C. pneumoniae genes encoding yersiniae T3S YscD, YscQ, and YscL homologs, respectively, were cloned and expressed as histidine- and glutathione S-transferase (GST)-tagged proteins in Escherichia coli. Purified recombinant proteins were used to raise hyper-immune polyclonal antiserum and were used in GST pull-down and copurification assays to identify protein-protein interactions. CdsD was detected in both EB and RB lysates by Western blot analyses, and immunofluorescent staining demonstrated the presence of CdsD within inclusions. Triton X-114 solubilization and phase separation of chlamydial EB proteins indicated that CdsD partitions with cytoplasmic proteins, suggesting it is not an integral membrane protein. GST pull-down assays indicated that recombinant CdsD interacts with CdsQ and CdsL, and copurification assays with chlamydial lysates confirmed that native CdsD interacts with CdsQ and CdsL. To the best of our knowledge, this is the first report demonstrating interactions between YscD, YscQ, and YscL homologs of bacterial T3S systems. These novel protein interactions may play important roles in the assembly or function of the chlamydial T3S apparatus.     

Identification of Novel Type III Secretion Chaperone-Substrate Complexes of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterial pathogen of humans that uses a type III secretion (T3S) system to manipulate host cells through the delivery of effector proteins into their cytosol and membranes. The function of T3S systems depends on small bacterial cytosolic chaperone-like proteins, which bind T3S substrates and ensure their appropriate secretion. To find novel T3S chaperone-substrate complexes of C. trachomatis we first searched its genome for genes encoding proteins with features of T3S chaperones. We then systematically tested for interactions between candidate chaperones and chlamydial T3S substrates by bacterial two-hybrid. This revealed interactions between Slc1 (a known T3S chaperone) or CT584 and several T3S substrates. Co-immunoprecipation after protein expression in Yersinia enterocolitica and protein overlay binding assays indicated that Slc1 interacted with the N-terminal region of the known T3S substrates Tarp (a previously described substrate of Slc1), CT694, and CT695, and that CT584 interacted with a central region of CT082, which we identified as a C. trachomatis T3S substrate using Y. enterocolitica as a heterologous system. Further T3S assays in Yersinia indicated that Slc1 or CT584 increased the amount of secreted Tarp, CT694, and CT695, or CT082, respectively. Expression of CT584 increased the intra-bacterial stability of CT082, while Slc1 did not affect the stability of its substrates. Overall, this indicated that in C. trachomatis Slc1 is a chaperone of multiple T3S substrates and that CT584 is a chaperone of the newly identified T3S substrate CT082.     

Disulfide bonding within components of the Chlamydia type III secretion apparatus correlates with development

Chlamydia spp. exhibit a unique biphasic developmental cycle whereby infectious elementary bodies (EBs) invade host epithelial cells and differentiate into noninfectious, metabolically active reticulate bodies (RBs). EBs posses a unique outer envelope where rigidity is achieved by disulfide bonding among cysteine-rich envelope-associated proteins. Conversely, these disulfide bonds become reduced in RBs to accommodate vegetative growth, thereby linking the redox status of cysteine-rich envelope proteins with progression of the developmental cycle. We investigated the potential role of disulfide bonding within the chlamydial type III secretion system (T3SS), since activity of this system is also closely linked to development. We focused on structural components of the T3S apparatus that contain an unusually high number of cysteine residues compared to orthologs in other secretion systems. Nonreducing SDS-PAGE revealed that EB-localized apparatus proteins such as CdsF, CdsD, and CdsC form higher-order complexes mediated by disulfide bonding. The most dramatic alterations were detected for the needle protein CdsF. Significantly, disulfide bonding patterns shifted during differentiation of developmental forms and were completely reduced in RBs. Furthermore, at later time points during infection following RB to EB conversion, we found that CdsF is reoxidized into higher-order complexes. Overall, we conclude that the redox status of specific T3SS apparatus proteins is intimately linked to the developmental cycle and constitutes a newly appreciated aspect of functionally significant alterations within proteins of the chlamydial envelope.     

Evidence that CT694 is a novel Chlamydia trachomatis T3S substrate capable of functioning during invasion or early cycle development

Chlamydia trachomatis is an obligate intracellular parasite, occupies a membrane-bound vacuole throughout development and is capable of manipulating the eukaryotic host by translocating effector molecules via a type III secretion system (T3SS). The infectious chlamydial elementary body (EB) is metabolically inactive yet possesses a functional T3S apparatus capable of translocating effector proteins into the host cell to facilitate invasion and other early cycle events. We present evidence here that the C. trachomatis protein CT694 represents an early cycle-associated effector protein. CT694 is secreted by the Yersinia T3SS and immunodetection studies of infected HeLa cultures indicate that CT694-specific signal accumulates directly adjacent to, but not completely overlapping with EBs during invasion. Yeast two-hybrid analyses revealed an interaction of CT694 with the repeat region and C-terminus of human AHNAK. Immunolocalization studies of CT694 ectopically expressed in HeLa cells were consistent with an interaction with endogenous AHNAK. Additionally, expression of CT694 in HeLa cells resulted in alterations in the detection of stress fibres that correlated with the ability of CT694 to interact with AHNAK. These data indicate that CT694 is a novel T3S-dependent substrate unique to C. trachomatis , and that its interaction with host proteins such as AHNAK may be important for aspects of invasion or development particular to this species.     

Secretion of Cpn0796 from Chlamydia pneumoniae into the host cell cytoplasm by an autotransporter mechanism

By comparison of proteome profiles of purified Chlamydia pneumoniae and whole lysates of C. pneumoniae infected HEp-2 cells, an N-terminal fragment of the previously uncharacterized chlamydial protein Cpn0796 was identified as a secreted protein. A 38 kDa cleavage product of Cpn0796 was present in infected cells, whereas only the 65 kDa full-length Cpn0796 could be detected in purified Chlamydia. Confocal immunofluorescence microscopy demonstrated that Cpn0796 was localized in the Chlamydia membrane in young inclusions. However, at 36 h post infection and later Cpn0796 was detected in the cytoplasm of C. pneumoniae infected HEp-2 and BHK cells. Furthermore, Cpn0796 was detected in the cytoplasm of infected cells in the lungs of C. pneumoniae infected C57Bl mice. When cleavage was inhibited, Cpn0796 was retained in the chlamydiae. We propose that Cpn0796 is an autotransporter the N-terminal of which is translocated to the host cell cytoplasm. This is the first example of secretion of a Chlamydia autotransporter passenger domain into the host cell cytoplasm. Cpn0796 is specific for C. pneumoniae, where five homologous proteins are encoded by clustered genes. None of these five proteins were found to be secreted.     

The chaperonin cycle and protein folding

The process of protein folding in the cell is now known to depend on the action of other proteins. These proteins include molecular chaperones, Which interact non-covalently with proteins as they fold and improve the final yields of active protein in the cell. The precise mechanism by which molecular chaperones act is obscure. Experiments reported recently⁽¹⁾ show that for one molecular chaperone (Cpn60, typified by the E. coli protein GroEL), the folding reaction is driven by cycles of binding and release of the co-chaperone Cpn10 (known as GroES in E. coli). These alternate with binding and release of the unfolded protein substrate. These cycles come about because of the opposite effects of Cpn10 and unfolded protein on the Cpn60 complex: the former stabilises the ADP-bound state of Cpn60, whereas the latter stimulates ADP-ATP exchange. This model proposes that the substrate protein goes through multiple cycles of binding and release, and is released into the cavity of the Cpn60 complex where it can undergo folding without interacting with other nearby folding intermediates. This is consistent with the ability of Cpn60 proteins to enhance folding by blocking pathways to aggregation.     

Bioinformatic and biochemical evidence for the identification of the type III secretion system needle protein of Chlamydia trachomatis

Chlamydia spp. express a functional type III secretion system (T3SS) necessary for pathogenesis and intracellular growth. However, certain essential components of the secretion apparatus have diverged to such a degree as to preclude their identification by standard homology searches of primary protein sequences. One example is the needle subunit protein. Electron micrographs indicate that chlamydiae possess needle filaments, and yet database searches fail to identify a SctF homologue. We used a bioinformatics approach to identify a likely needle subunit protein for Chlamydia. Experimental evidence indicates that this protein, designated CdsF, has properties consistent with it being the major needle subunit protein. CdsF is concentrated in the outer membrane of elementary bodies and is surface exposed as a component of an extracellular needle-like projection. During infection CdsF is detectible by indirect immunofluorescence in the inclusion membrane with a punctuate distribution adjacent to membrane-associated reticulate bodies. Biochemical cross-linking studies revealed that, like other SctF proteins, CdsF is able to polymerize into multisubunit complexes. Furthermore, we identified two chaperones for CdsF, termed CdsE and CdsG, which have many characteristics of the Pseudomonas spp. needle chaperones PscE and PscG, respectively. In aggregate, our data are consistent with CdsF representing at least one component of the extended Chlamydia T3SS injectisome. The identification of this secretion system component is essential for studies involving ectopic reconstitution of the Chlamydia T3SS. Moreover, we anticipate that CdsF could serve as an efficacious target for anti-Chlamydia neutralizing antibodies.     

Effector proteins of chlamydiae

This review summarizes the recently published data on the molecular mechanisms of Chlamydiae-host cell interaction, first of all, on chlamydial effector proteins. Such proteins, along with type III transport system proteins, which transfer many effector proteins into the host cytoplasm, are attractive targets for drug therapy of chlamydial infections. The majority of the data concerns two species, Chlamydia trachomatis and Chlamydophila pneumoniae. The C. trachomatis protein TARP, which is presynthesized in elementary bodies, plays an essential role in the initial stages of infection. The pathogen proteins that are involved in the next stage, which is the intracellular inclusion traffic to the centrosome, are C. trachomatis CT229 and C. pneumoniae Cpn0585, which interact with cell Rab GTPases. In C. trachomatis, IncA plays a key role in the fusion of chlamydial inclusions, CT847 modulates the life cycle of the host cell, and LDA3 is essential for the acquisition of nutrients. The protease CPAF and the inclusion membrane proteins IncG and CADD are involved in suppressing apoptosis of infected cells. The proteases CPAF and CT441 and the deubiquitinating protein ChlaDub1 help the pathogen to evade the immune response.     

An inclusion membrane protein from Chlamydia trachomatis enters the MHC class I pathway and stimulates a CD8+ T cell response

During its developmental cycle, the intracellular bacterial pathogen Chlamydia trachomatis remains confined within a protective vacuole known as an inclusion. Nevertheless, CD8(+) T cells that recognize Chlamydia Ags in the context of MHC class I molecules are primed during infection. MHC class I-restricted presentation of these Ags suggests that these proteins or domains from them have access to the host cell cytoplasm. Chlamydia products with access to the host cell cytoplasm define a subset of molecules uniquely positioned to interface with the intracellular environment during the pathogen's developmental cycle. In addition to their use as candidate Ags for stimulating CD8(+) T cells, these proteins represent novel candidates for therapeutic intervention of infection. In this study, we use C. trachomatis-specific murine T cells and an expression-cloning strategy to show that CT442 from Chlamydia is targeted by CD8(+) T cells. CT442, also known as CrpA, is a 15-kDa protein of undefined function that has previously been shown to be associated with the Chlamydia inclusion membrane. We show that: 1) CD8(+) T cells specific for an H-2D(b)-restricted epitope from CrpA are elicited at a significant level (approximately 4% of splenic CD8(+) T cells) in mice in response to infection; 2) the response to this epitope correlates with clearance of the organism from infected mice; and 3) immunization with recombinant vaccinia virus expressing CrpA elicits partial protective immunity to subsequent i.v. challenge with C. trachomatis.     

HpaC controls substrate specificity of the Xanthomonas type III secretion system

The Gram-negative bacterial plant pathogen Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to inject bacterial effector proteins into the host cell cytoplasm. One essential pathogenicity factor is HrpB2, which is secreted by the T3S system. We show that secretion of HrpB2 is suppressed by HpaC, which was previously identified as a T3S control protein. Since HpaC promotes secretion of translocon and effector proteins but inhibits secretion of HrpB2, HpaC presumably acts as a T3S substrate specificity switch protein. Protein-protein interaction studies revealed that HpaC interacts with HrpB2 and the C-terminal domain of HrcU, a conserved inner membrane component of the T3S system. However, no interaction was observed between HpaC and the full-length HrcU protein. Analysis of HpaC deletion derivatives revealed that the binding site for the C-terminal domain of HrcU is essential for HpaC function. This suggests that HpaC binding to the HrcU C terminus is key for the control of T3S. The C terminus of HrcU also provides a binding site for HrpB2; however, no interaction was observed with other T3S substrates including pilus, translocon and effector proteins. This is in contrast to HrcU homologs from animal pathogenic bacteria suggesting evolution of distinct mechanisms in plant and animal pathogenic bacteria for T3S substrate recognition.     

NF-kappaB and inhibitor of apoptosis proteins are required for apoptosis resistance of epithelial cells persistently infected with Chlamydophila pneumoniae

Infection with Chlamydophila pneumoniae (Cpn) renders host cells resistant to apoptosis induced by a variety of stimuli. While modulation of apoptosis has been extensively studied in cells acutely infected with Cpn, very little is known on how persistent chlamydial infection influences host cell survival. Here we show that epithelial cells persistently infected with Cpn resist apoptosis induced with TNFalpha or staurosporine. Cpn induced the activation of nuclear factor kappa B (NF-kappaB) and inhibition of NF-kappaB with a chemical inhibitor or by silencing expression of the p65 subunit sensitized infected cells for apoptosis induction by staurosporine or TNFalpha. Persistent infection resulted in the upregulation of the NF-kappaB regulated inhibitor of apoptosis protein 2 (cIAP-2) but not inhibitor of apoptosis protein 1 (cIAP-1). Interestingly, silencing of either cIAP-1 or cIAP-2 sensitized infected cells, suggesting that IAPs play an important role in the apoptosis resistance of persistently infected cells.     

Identification of a receptor-binding domain of the spike glycoprotein of human coronavirus HCoV-229E

Human coronavirus HCoV-229E uses human aminopeptidase N (hAPN) as its receptor (C. L. Yeager et al., Nature 357:420-422, 1992). To identify the receptor-binding domain of the viral spike glycoprotein (S), we expressed soluble truncated histidine-tagged S glycoproteins by using baculovirus expression vectors. Truncated S proteins purified by nickel affinity chromatography were shown to be glycosylated and to react with polyclonal anti-HCoV-229E antibodies and monoclonal antibodies to the viral S protein. A truncated protein (S(547)) that contains the N-terminal 547 amino acids bound to 3T3 mouse cells that express hAPN but not to mouse 3T3 cells transfected with empty vector. Binding of S(547) to hAPN was blocked by an anti-hAPN monoclonal antibody that inhibits binding of virus to hAPN and blocks virus infection of human cells and was also blocked by polyclonal anti-HCoV-229E antibody. S proteins that contain the N-terminal 268 or 417 amino acids did not bind to hAPN-3T3 cells. Antibody to the region from amino acid 417 to the C terminus of S blocked binding of S(547) to hAPN-3T3 cells. Thus, the data suggest that the domain of the spike protein between amino acids 417 and 547 is required for the binding of HCoV-229E to its hAPN receptor.     

Akt suppresses DLK for maintaining self-renewal of mouse embryonic stem cells

Mouse embryonic stem cells (ES cells) can proliferate indefinitely. To identify potential signals involved in suppression of self-renewal, we previously screened a kinase/phosphatase expression library in ES cells, and observed that inhibition of Dual Leucine zipper-bearing Kinase (DLK) increased relative cell numbers. DLK protein was detected in both the pluripotent and differentiated states of mouse ES cells while DLK kinase activity increased upon differentiation. Overexpression of DLK in mouse ES cells displayed reductions in relative cell/colony numbers and Nanog expression, suggesting a suppressive role of DLK in self-renewal. By examining protein sequences of DLK, we identified 2 putative Akt phosphorylation sites at S584 and T659. Blocking PI3K/Akt signaling with LY-294002 enhanced DLK kinase activity dramatically. We found that Akt interacts with and phosphorylates DLK. Mutations of DLK amino acid residues at putative Akt phosphorylation sites (S584A, T659A, or S584A and T659A) diminished the level of DLK phosphorylation. While the mutated DLKs (S584A, T659A, or S584A and T659A) were expressed, a further reduction in cell/colony numbers and Nanog expression appeared in mouse ES cells. In addition, these mutant DLKs (S584A, T659A, or S584A and T659A) exhibited more robust kinase activity and cell death compared to wild type DLK or green fluorescence (GFP) controls. In summary, our results show that DLK functions to suppress self-renewal of mouse ES cells and is restrained by Akt phosphorylation.     

Identification of Chlamydia trachomatis CT621, a protein delivered through the type III secretion system to the host cell cytoplasm and nucleus

Chlamydiae are obligate intracellular bacteria, developing inside host cells within chlamydial inclusions. From these inclusions, the chlamydiae secrete proteins into the host cell cytoplasm. A pathway through which secreted proteins can be delivered is the type III secretion system (T3SS). The T3SS is common to several gram-negative bacteria and the secreted proteins serve a variety of functions often related to the modulation of host signalling. To identify new potentially secreted proteins, the cytoplasm was extracted from Chlamydia trachomatis L2-infected HeLa cells, and two-dimensional polyacrylamide gel electrophoresis profiles of [³⁵S]-labelled chlamydial proteins from this extract were compared with profiles of chlamydial proteins from the lysate of infected cells. In this way, CT621 was identified. CT621 is a member of a family of proteins containing a domain of unknown function DUF582 that is only found within the genus Chlamydia . Immunofluorescence microscopy and immunoblotting demonstrated that CT621 is secreted late in the chlamydial developmental cycle and that it is the first chlamydial protein found to be localized within both the host cell cytoplasm and the nucleus. To determine whether CT621 is secreted through the T3SS, an inhibitor of this apparatus was added to the infection medium, resulting in retention of the protein inside the chlamydiae. Hence, the so far uncharacterized CT621 is a new type III secretion effector protein.     

Functional consequences of single:double ring transitions in chaperonins: life in the cold

The cpn60 and cpn10 genes from psychrophilic bacterium, Oleispira antarctica RB8, showed a positive effect in Escherichia coli growth at low temperature, shifting its theoretical minimal growth temperature from +7.5 degrees C to -13.7 degrees C [Ferrer, M., Chernikova, T.N., Yakimov, M., Golyshin, P.N., and Timmis, K.N. (2003) Nature Biotechnol 21: 1266-1267]. To provide experimental support for this finding, Cpn60 and 10 were overproduced in E. coli and purified to apparent homogeneity. Recombinant O.Cpn60 was identical to the native protein based on tetradecameric structure, and it dissociates during native PAGE. Gel filtration and native PAGE revealed that, in vivo and in vitro, (O.Cpn60)(7) was the active oligomer at 4-10 degrees C, whereas at > 10 degrees C, this complex was converted to (O.Cpn60)(14). The dissociation reduces the ATP consumption (energy-saving mechanism) and increases the refolding capacity at low temperatures. In order for this transition to occur, we demonstrated that K468 and S471 may play a key role in conforming the more advantageous oligomeric state in O.Cpn60. We have proved this hypothesis by showing that single and double mutations in K468 and S471 for T and G, as in E.GroEL, produced a more stable double-ring oligomer. The optimum temperature for ATPase and chaperone activity for the wild-type chaperonin was 24-28 degrees C and 4-18 degrees C, whereas that for the mutants was 45-55 degrees C and 14-36 degrees C respectively. The temperature inducing unfolding (T(M)) increased from 45 degrees C to more than 65 degrees C. In contrast, a single ring mutant, O.Cpn60(SR), with three amino acid substitutions (E461A, S463A and V464A) was as stable as the wild type but possessed refolding activity below 10 degrees C. Above 10 degrees C, this complex lost refolding capacity to the detriment of the double ring, which was not an efficient chaperone at 4 degrees C as the single ring variant. We demonstrated that expression of O.Cpn60(WT) and O.Cpn60(SR) leads to a higher growth of E. coli at 4 degrees C ( micro (max), 0.22 and 0.36 h(-1) respectively), whereas at 10-15 degrees C, only E. coli cells expressing O.Cpn60 or O.Cpn60(DR) grew better than parental cells (-cpn). These results clearly indicate that the single-to-double ring transition in Oleispira chaperonin is a wild-type mechanism for its thermal acclimation. Although previous studies have also reported single-to-double ring transitions under many circumstances, this is the first clear indication that single-ring chaperonins are necessary to support growth when the temperature falls from 37 degrees C to 4 degrees C.     

The chemoattractant receptor-like protein C5L2 binds the C3a des-Arg77/acylation-stimulating protein

The orphan receptor C5L2 has recently been described as a high affinity binding protein for complement fragments C5a and C3a that, unlike the previously described C5a receptor (CD88), couples only weakly to G(i)-like G proteins (Cain, S. A., and Monk, P. N. (2002) J. Biol. Chem. 277, 7165-7169). Here we demonstrate that C5L2 binds the metabolites of C4a and C3a, C4a des-Arg(77), and C3a des-Arg(77) (also known as the acylation-stimulating protein or ASP) at a site distinct from the C5a binding site. The binding of these metabolites to C5L2 does not stimulate the degranulation of transfected rat basophilic leukemia cells either through endogenous rat G proteins or when co-transfected with human G(alpha 16). C3a des-Arg(77)/ASP and C3a can potently stimulate triglyceride synthesis in human skin fibroblasts and 3T3-L1 preadipocytes. Here we show that both cell types and human adipose tissue express C5L2 mRNA and that the human fibroblasts express C5L2 protein at the cell surface. This is the first demonstration of the expression of C5L2 in cells that bind and respond to C3a des-Arg(77)/ASP and C3a. Thus C5L2, a promiscuous complement fragment-binding protein with a high affinity site that binds C3a des-Arg(77)/ASP, may mediate the acylation-stimulating properties of this peptide.     

An insulin-stimulated ribosomal protein S6 kinase in 3T3-L1 cells

A protein kinase that is stimulated from 2-10-fold by insulin and that phosphorylates ribosomal protein S6 has been characterized in 3T3-L1 cells. The detection of this activity in the 100,000 X g supernatant is facilitated by the presence of beta-glycerol phosphate or vanadate in the homogenization buffer. The activity has been purified 55-fold by chromatography on DEAE-cellulose and phosphocellulose. The resulting specific activity is 584 pmol/min/mg of protein. DEAE-cellulose chromatography followed by gel filtration on Ultrogel AcA54 or by glycerol gradient centrifugation suggests that the protein has a molecular mass of 60,000-70,000 daltons. Mg2+, and to a lesser extent Mn2+, will support phosphorylation of S6 by the activity. No proteins tested other than ribosomal protein S6 are phosphorylated. Based on its chromatographic properties and substrate specificity, the enzyme appears to be distinct from several other protein kinases that are known to phosphorylate ribosomal protein S6 in vitro. The complete characterization and purification of this enzyme may be essential to the elucidation of the mechanism of regulation of S6 phosphorylation by insulin.     

Low temperature-induced systems failure in Escherichia coli: insights from rescue by cold-adapted chaperones

The growth of Escherichia coli cells is impaired at temperatures below 21 degrees C and stops at 7.5 degrees C; however, growth of a transgenic strain producing the cold-adapted chaperones Cpn60 and Cpn10 from the psychrophilic bacterium Oleispira antarctica is good at low temperatures. The E. coli cpn(+) transgene offers a novel opportunity for examining the essential protein for cell viability at low temperatures. By screening a large-scale protein map (proteome) of cells of K-12 and its Cpn(+) transgene incubated at 4 degrees C, we identified 22 housekeeping proteins involved in systems failure of E. coli when confronted with low temperature. Through co-immunoprecipitation of Cpn60, Northern blot, and in vitro refolding, we systematically identified that protein-chaperone interactions are key determinants of their protein functions at low temperatures. Furthermore, chromosomal gene deletion experiments suggest that the mechanism of cold-induced systems failure in E. coli is cold-induced inactivation of the GroELS chaperonins and the resulting failure to refold cold-inactivated Dps, ClpB, DnaK and RpsB proteins. These findings: (1) indicate the potential importance of chaperones in cold sensitivity, cold adaptation and cold tolerance in cellular systems, and (2) suggest the identity of a few key cold-sensitive chaperone-interacting proteins that get inactivated and ultimately cause systems failure in E. coli cells at low temperatures.     

HpaA from Xanthomonas is a regulator of type III secretion

The Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to inject effector proteins into the host cell cytoplasm. Efficient secretion of several effector proteins depends on the cytoplasmic global T3S chaperone HpaB. In this study, we show that HpaB interacts with the virulence factor HpaA, which is secreted by the T3S system and translocated into the plant cell. HpaA promotes secretion of pilus, translocon and effector proteins and therefore appears to be an important control protein of the T3S system. Protein-protein interaction studies and the analysis of HpaA deletion derivatives revealed that the C-terminal protein region, which contains a HpaB binding site, is crucial for the contribution of HpaA to T3S. Secretion of pilus and translocon proteins is not affected when HpaA is expressed as an N-terminal deletion derivative that lacks the secretion and translocation signal. Our data suggest that binding of HpaA to HpaB within the bacterial cell favours secretion of extracellular components of the secretion apparatus. Secretion of HpaA presumably liberates HpaB and thus promotes effector protein secretion after assembly of the T3S apparatus.