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Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
The Cpx Envelope Stress Response, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap05-1.gif /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap05-2.gifAbstract:
This chapter summarizes studies that led to identification and characterization of the Cpx envelope stress response and highlights recent work that hints at a diverse range of Cpx-influenced cellular phenotypes. Silverman and colleagues provided the kernels for our current understanding of the Cpx envelope stress response. The Cpx envelope stress response is regulated by a typical two-component regulatory system. In addition to functioning as an activator and a repressor, a new role for CpxR~P in transcription has recently been put forth, that of potentiator. Researchers have shown by microarray analysis of biofilm and planktonic populations of cells that the Cpx-regulated genes cpxP and spy were highly up-regulated in biofilms and that cpxP and cpxR mutants formed biofilms with reduced mass and substrate coverage. It has been shown that CpxR homologues in Legionella pneumophila and Yersinia enterocolitica are likely involved in transcription of the icm-dot genes encoding the type IV secretion system and degP, respectively, both of which are necessary for virulence. Analysis of TraJ, TraY, and TraM protein half-lives in wild-type and Cpx-induced strains indicated that TraJ stability was specifically decreased when the Cpx response was activated. This study was the first to indicate that the Cpx response might influence the fates of intracellular proteins, in addition to those found in the envelope. The Cpx envelope stress response exhibits significant connectivity to other adaptive responses. Although it has become dogma that CpxA senses and is activated by misfolded envelope proteins, the mechanism(s) involved remain elusive.
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Independent genetic selections identified the Cpx locus. (Left) Silverman and colleagues selected conjugal F plasmid donors (right rectangle) that were resistant to the Qβ phage (hexagon), which adsorbs to the F pilus (thin grey line connecting donor and recipient cells). These mutants were defective in conjugal DNA transfer to F-recipient cells (left rectangle) and mapped to the cpxA locus. (Right) Silhavy and colleagues selected for mutants resistant to the expression of the toxic, misfolded, mislocalized proteins LamBA23D (rectangle joined to squiggly line) or a tribrid fusion protein LamB-LacZ-PhoA (squiggly line). LamBA23D contains a signal-sequence mutation that prevents cleavage of the signal sequence by leader peptidase (oval) and causes slowed processing, altered localization, and sensitivity to the inducer maltose (MalS) and SDS (SDSS). LamB-LacZ-PhoA leads to the production of disulfide-bonded aggregates of β-galactosidase in the periplasm (squiggly line), which is manifest as sensitivity to the inducer maltose (MalS). SDSR or MalR mutants mapped to the cpxA locus. OM, outer membrane; PP, periplasm; IM, inner membrane.
Cpx signal transduction is mediated by a two-component regulatory system. Envelope stresses are sensed by an inner membrane-localized sensor histidine kinase, CpxA. In the absence of envelope stress, CpxA functions as a CpxR~P (light shaded rectangle) phosphatase (dark shaded oval). In the presence of envelope stress, CpxA is thought to undergo a conformational change (dark shaded rectangle), which causes it to take on autokinase and CpxR (light shaded oval) kinase activities. Phos-phorylation of CpxR converts it to a transcription factor able to bind with increased affinity to the promoters of target genes. OM, outer membrane; PP, periplasm; IM, inner membrane; Pi, inorganic phosphate; H, histidine; D, aspartate; P, phosphate.
Cpx-inducing cues and -signaling proteins. All Cpx-inducing cues are sensed via the periplasmic sensing domain of CpxA (small shaded rectangle). Attachment to abiotic surfaces is signaled first through the outer membrane lipoprotein NlpE (shaded rectangle). Most other Cpx-inducing cues (boxed at bottom left) do not require NlpE for signaling. Some of these (pH, H+/OH-; P pilus subunit overexpression) lead to DegP-depen-dent degradation of the inhibitory signaling protein CpxP (light shaded oval in periplasm). Additional growth-related cues likely enter the signaling pathway downstream of CpxA but upstream of CpxR. PP, periplasm; IM, inner membrane; CYTO, cytoplasm; BFP, bundle-forming pilus.
The Cpx regulon