A sporulation membrane protein tethers the pro-sigmaK processing enzyme to its inhibitor and dictates its subcellular localization

Forespore signaling is necessary for pro-sigmaK processing during Bacillus subtilis sporulation despite the loss of SpoIVFA upon translational arrest

The sigmaK checkpoint coordinates gene expression in the mother cell with signaling from the forespore during Bacillus subtilis sporulation. The signaling pathway involves SpoIVB, a serine peptidase produced in the forespore, which is believed to cross the innermost membrane surrounding the forespore and activate a complex of proteins, including BofA, SpoIVFA, and SpoIVFB, located in the outermost membrane surrounding the forespore. Activation of the complex allows proteolytic processing of pro-sigmaK, and the resulting sigmaK RNA polymerase transcribes genes in the mother cell. To investigate activation of the pro-sigmaK processing complex, the level of SpoIVFA in extracts of sporulating cells was examined by Western blot analysis. The SpoIVFA level decreased when pro-sigmaK processing began during sporulation. In extracts of a spoIVB mutant defective in forespore signaling, the SpoIVFA level failed to decrease normally and no processing of pro-sigmaK was observed. Although these results are consistent with a model in which SpoIVFA inhibits processing until the SpoIVB-mediated signal is received from the forespore, we discovered that loss of SpoIVFA was insufficient to allow processing under certain conditions, including static incubation of the culture and continued shaking after the addition of inhibitors of oxidative phosphorylation or translation. Under these conditions, loss of SpoIVFA was independent of spoIVB. The inability to process pro-sigmaK under these conditions was not due to loss of SpoIVFB, the putative processing enzyme, or to a requirement for ongoing synthesis of pro-sigmaK. Rather, it was found that the requirements for shaking of the culture, for oxidative phosphorylation, and for translation could be bypassed by mutations that uncouple processing from dependence on forespore signaling. This suggests that ongoing translation is normally required for efficient pro-sigmaK processing because synthesis of the SpoIVB signal protein is needed to activate the processing complex. When translation is blocked, synthesis of SpoIVB ceases, and the processing complex remains inactive despite the loss of SpoIVFA. Taken together, the results suggest that SpoIVB signaling activates the processing complex by performing another function in addition to causing loss of SpoIVFA or by causing loss of SpoIVFA in a different way than when translation is blocked. The results also demonstrate that the processing machinery can function in the absence of translation or an electrochemical gradient across membranes.

Evidence that subcellular localization of a bacterial membrane protein is achieved by diffusion and capture

Bacteria lack an endoplasmic reticulum, a Golgi apparatus, and transport vesicles and yet are capable of sorting and delivering integral membrane proteins to particular sites within the cell with high precision. What is the pathway by which membrane proteins reach their proper subcellular destination in bacteria? We have addressed this question by using green fluorescent protein (GFP) fused to a polytopic membrane protein (SpoIVFB) that is involved in the process of sporulation in the bacterium Bacillus subtilis. SpoIVFB-GFP localizes to a region of the sporulating cell known as the outer forespore membrane, which is distinct from the cytoplasmic membrane. Experiments are presented that rule out a mechanism in which SpoIVFB-GFP localizes to all membranes but is selectively eliminated from the cytoplasmic membrane by proteolytic degradation and argue against a model in which SpoIVFB-GFP is selectively inserted into the outer forespore membrane. Instead, the results are most easily compatible with a model in which SpoIVFB-GFP achieves proper localization by insertion into the cytoplasmic membrane followed by diffusion to, and capture in, the outer forespore membrane. The possibility that diffusion and capture is a general feature of protein localization in bacteria is discussed.