Accumulation of secretory protein precursors in Escherichia coli induces the heat shock response

Abstract

The accumulation of secretory protein precursors, caused either by mutations in secB or secA or by the overproduction of export-defective proteins, results in a two-to fivefold increase in the synthesis of heat shock proteins. In such strains, &.32, the alternative sigma factor responsible for transcription of the heat shock genes, is stabilized. The resultant increase in the level of &32 leads to increased transcription of heat shock genes and increased synthesis of heat shock proteins. We have also found that although a secB null mutant does not grow on rich medium at a temperature range of 30 to 42°C, it does grow at 44°C. In addition, we found that a secB null mutant exhibits greater thermotolerance than the wild-type parental strain. Elevated levels of heat shock proteins, as well as some other non-heat shock proteins, may account for the partial heat resistance of a SecB-lacking strain.The Escherichia coli SecB protein participates in the export of a subset of proteins destined for the periplasm or outer membrane. In its function as a chaperone, it maintains target precursor proteins in a partially unfolded state competent for translocation. Moreover, because of its interaction with SecA, the peripheral membrane domain of the translocase, SecB helps to position precursor proteins at membrane sites containing the integral SecY/E components of translocase (for a review, see references 24, 25, and 30). Strains lacking the SecB chaperone are viable, but they have a number of pleiotropic defects. In such strains, export of the secretory proteins LamB, OmpA, OmpF, PhoE, and maltose-binding protein is defective, leading to an accumulation of precursor forms of these proteins. In addition, although secB deletion strains are viable on minimal glycerol medium, they are unable to grow on rich medium (16, 17).We have recently shown that the DnaK and DnaJ heat shock proteins can partially substitute for the SecB chaperone function. Strains lacking SecB require DnaK and DnaJ both for residual transport of several SecB-dependent proteins and for viability on minimal medium. Furthermore, overproduction of DnaK and DnaJ permits secB null mutants to grow on rich medium, albeit at a reduced rate, and enhances the rate of export of some SecB-dependent proteins (31). These observations led us to wonder whether mutants lacking SecB might have a higher level of heat shock proteins than wild-type strains. This seemed possible because the cytoplasmic precursor proteins that accumulate in secB deletion strains may be seen as abnormal proteins by the cell. A variety of abnormal proteins, including puromycyl fragments (11), induce the heat shock response. In addition, unfolded proteins that are not degraded (23) and the MalE-LacZ hybrid protein (14) also induce the heat shock response. In this report, we show that in a secB null mutant and a secA(Ts) mutant, synthesis of heat shock proteins is also induced. Moreover, we show that the signal for induction of heat shock protein synthesis is accumulation of cytoplasmic precursors of sec...

Analysis of σ 32 mutants defective in chaperone-mediated feedback control reveals unexpected complexity of the heat shock response

“…All strains used were derivatives of E. coli K-12 strain MG1655. 32 mutants originally isolated on pRB11 or slightly different rpoH plasmids were placed under the control of an IPTG-inducible promoter in strain CAG48238 carrying ⌬lacX74 and prophage JW2 (P htpG -lacZ) (31) and its ⌬rpoH derivative.…”

Identification of a Region Required for Binding to Presecretory Protein in Bacillus Subtilis Ffh, A Homologue of the 54‐kDa Subunit of Mammalian Signal Recognition Particle

“…folded state by molecular chaperones (Kumamoto, 1989 ;Wild et al, 1992Wild et al, , 1993. They are then recognized by SecA and translocated across the cytoplasmic membrane through an ATP-dependent translocase consisting of SecA, SecE, SecG, SecY and other membrane proteins (Lill et al, 1989;Akita et al, 1990;Economou and Wickner, 1994;Economou et al, 1995).…”

Dynamic interplay between antagonistic pathways controlling the σ 32 level in Escherichia coli

“…The initial stabilization probably results from sequestering 32 away from the DnaK-DnaJ chaperones because of heat-induced accumulation of unfolded or misfolded proteins and facilitating 32 to bind core RNA polymerase, whereas subsequent destabilization results from accumulation of DnaKDnaJ chaperones and proteases caused by the increase in 32 (9)(10)(11). Accumulation of abnormal proteins without temperature upshift induces HSP synthesis (12) through stabilization but not induced synthesis of 32 (13,14). Although the membranebound ATP-dependent metalloprotease FtsH (HflB) was first shown to be responsible for rapid turnover of 32 (15,16), a set of cytosolic proteases including HslVU (ClpYQ) also participate in degradation of 32 in vivo and in vitro (17,18).…”