Two proteolytic modules are involved in regulated intramembrane proteolysis of Bacillus subtilis RsiW

Stress-induced degradation of the Bacillus subtilis anti-sigma factor RsiW results in the induction of genes controlled by the extracytoplasmic function sigma factor sigma(W). RsiW is cleaved by the mechanism of regulated intramembrane proteolysis at site-1 and -2 by PrsW and RasP respectively, and is then further degraded by cytoplasmic Clp peptidases. In a reconstituted Escherichia coli system, PrsW removes 40 amino acids from RsiW by cleaving between Ala168 and Ser169 of the extracytoplasmic domain, thereby generating RsiW-S1. Further trimming of RsiW-S1's C-terminus by the periplasmic tail-specific protease Tsp is crucial for subsequent RasP-catalysed clipping. In B. subtilis, mutation of RsiW at Ala168 severely impairs site-1 processing. RsiW-S1 is undetectable in wild-type B. subtilis and knockout strains lacking various extracytoplasmic proteases. While it can be stabilized by C-terminal tagging, even this fusion protein is still attacked. Thus, several peptidases seem to be involved in trimming of RsiW downstream of PrsW and upstream of RasP in B. subtilis. Overall, the RsiW degradation pathway can be subdivided into two modules each consisting of a site-specific peptidase that prepares RsiW for further degradation by downstream proteases.