Direct CIII–HflB interaction is responsible for the inhibition of the HflB (FtsH)‐mediated proteolysis of Escherichia coliσ³² by λCIII

Abstract

Development of the temperate coliphage k depends on a set of phage-specified regulatory proteins that interact with host target proteins [1][2][3][4]. Among several effects on host protein synthesis, k provokes the overproduction of some bacterial proteins induced in the heat shock response. This effect depends on early gene expression encoded by the leftward (p L ) transcription unit of k [5,6]. CIII is a k-specific regulatory protein from the p L operon with a potential for host interaction. It is a small 54-residue protein that favors the lysogenic response to infection by stabilizing kCII, the transcriptional factor that favors lysogeny and is responsible for primary control of the lambda developmental decision for lysis or lysogeny [4,[7][8][9]. In the absence of CIII, CII is rapidly degraded by the ATPdependent host metalloprotease HflB (FtsH). The molecular mechanism for CIII-mediated inhibition of the proteolysis of CII by HflB involves CIII-HflB interaction [10,11]. CIII itself is a substrate of HflB [12]. It has also been reported that CIIIC, the central helical domain of CIII, is resistant to HflB proteolysis, and is a more effective inhibitor than the full-length protein [11].Apart from promoting lysogeny by lambda, CIII protects Escherichia coli r 32 , the heat shock-specific sigma factor, which is also a substrate of HflB [13,14]. This effect generates a heat-shock response in the cell [15,16]. Maximal production of CIII prolongs the heat-induced synthesis of E. coli heat shock proteins even at low temperature [17]. The half-life of r 32 ( 2 min) is increased approximately fourfold upon overproduction of CIII, resulting in an overproduction of heat shock proteins and rapid inhibition of cell growth [15]. The molecular mechanism of the CIIImediated inhibition of proteolysis of r 32 by HflB is unknown.We tried to address this issue by studying the effects of CIII and CIIIC on the proteolysis of

Small Proteins Can No Longer Be Ignored

“…The third function of SpoVM during sporulation is not well understood, but stems from the observation that it is a competitive inhibitor of the membrane-bound protease FtsH (21) and shares a limited amino acid sequence homology with the cIII protein of bacteriophage lambda (22), another small protein that inhibits FtsH in the Gram-negative E. coli (23). While FtsH is required for entry into sporulation (24), the role that SpoVM may play in inhibiting FtsH later during the sporulation program is not known.…”

Studies on Escherichia coliHflKC suggest the presence of an unidentified λ factor that influences the lysis-lysogeny switch

“…Thus it appears that some additional factors, supplied by the infecting phage, caused a stabilization of CII in the absence of HflKC. The only known phage factor that favors lysogeny by inhibiting the proteolysis of CII by HflB, is CIII [29,36]. We therefore tested the possible involvement of CIII as the λ factor responsible for the above result, viz.…”

“…CIII is a general inhibitor of CII proteolysis [29,36,37]. It is therefore expected that between a wild type strain alone and one that carries CIII, CII would exhibit a greater stability in the latter.…”

“…Plasmid pC2C3 (containing the cII and cIII genes) was constructed in three steps. First, the Nde I- Bam HI fragment of pAB905 containing the cIII gene [29] was cloned into pKP07 [28] and was named pLaCIII (containing the cIII gene under LacP). Then the Bgl II- Xho I fragment of pLaCIII (i.e.…”