Fine-tuning of the Escherichia coli σ^E envelope stress response relies on multiple mechanisms to inhibit signal-independent proteolysis of the transmembrane anti-sigma factor, RseA

Abstract

Proteolytic cascades are widely implicated in signaling between cellular compartments. In Escherichia coli, accumulation of unassembled outer membrane porins (OMPs) in the envelope leads to expression of E -dependent genes in the cytoplasmic cellular compartment. A proteolytic cascade conveys the OMP signal by regulated proteolysis of RseA, a membrane-spanning anti-sigma factor whose cytoplasmic domain inhibits E -dependent transcription. Upon activation by OMP C termini, the membrane localized DegS protease cleaves RseA in its periplasmic domain, the membrane-embedded protease RseP (YaeL) cleaves RseA near the inner membrane, and the released cytoplasmic RseA fragment is further degraded. Initiation of RseA degradation by activated DegS makes the system sensitive to a wide range of OMP concentrations and unresponsive to variations in the levels of DegS and RseP proteases. These features rely on the inability of RseP to cleave intact RseA. In the present report, we demonstrate that RseB, which binds to the periplasmic face of RseA, and DegS each independently inhibits RseP cleavage of intact RseA. Thus, the function of RseB, widely conserved among bacteria using the E pathway, and the second role of DegS (in addition to RseA proteolysis initiation) is to improve the performance characteristics of this signal transduction system.

Design principles of the proteolytic cascade governing the σE-mediated envelope stress response in Escherichia coli: keys to graded, buffered, and rapid signal transduction

“…The mechanistic details underlying this are unknown; however DegS, RseB, the periplasmic domain of RseA, and the PDZ domain of RseP participate in at least two independent mechanisms to repress activity of this protease toward full-length RseA (Kanehara et al 2003;Bohn et al 2004;Grigorova et al 2004). We have suggested that RseP cleavage of full-length RseA is blocked so that DegS cleavage of RseA will be rate-limiting over a wide range of OMP signal.…”

“…However, when porin C termini bind to the DegS PDZ domain, the catalytic triad is repositioned and cleavage of RseA ensues (Walsh et al 2003;Wilken et al 2004). DegS, RseB, RseA, and the PDZ domain of RseP itself all inhibit RseP cleavage of intact RseA, thereby ensuring that DegS always acts before RseP in the RseAprocessing pathway (Kanehara et al 2003;Bohn et al 2004;Grigorova et al 2004). Since DegS is the only protease that senses the porin signal, obligate sequential cleavage of RseA ensures that the initiation of the proteolytic cascade reflects the status of this inducing signal.…”

The atypical two-subunit σ factor from Bacillus subtilis is regulated by an integral membrane protein and acid stress

“…Under environmental conditions that elevate ECF s factor activity, many integral membrane anti-s factors undergo RIP, as is the case for anti-s factors regulating s E from E. coli and s W and s V from B. subtilis (Chaba et al, 2007;Grigorova et al, 2004;Hastie et al, 2014;Ho & Ellermeier, 2012). RIP is initiated by a highly specific site-1-protease (S1P).…”

Solution structures of RseA and its complex with RseB

“…This sequential digestion of RseA results in the release of E , which ultimately enhances the transcription of the gene involved in stress response (Alba et al, 2002;Kanehara et al, 2002). RseB prevents the proteolytic cleavage of RseA by binding to the periplasmic region of RseA (Missiakas et al, 1997;Grigorova et al, 2004). It is thought that the role of RseB is essential for the negative regulation of E signalling, since RseP alone can cleave RseA in a cell in which RseB and DegS are null-mutated (Grigorova et al, 2004).…”

“…RseB prevents the proteolytic cleavage of RseA by binding to the periplasmic region of RseA (Missiakas et al, 1997;Grigorova et al, 2004). It is thought that the role of RseB is essential for the negative regulation of E signalling, since RseP alone can cleave RseA in a cell in which RseB and DegS are null-mutated (Grigorova et al, 2004). RseB has also been proposed to activate the function of E by sensing other stress signals, including damaged proteins in periplasmic space.…”