Subcellular protein localization in B. subtilis

Division Of Basic Sciences - Nci

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Abstract

Advances in cytological methods over the last fifteen years have revealed that bacteria, despite the absence of membrane-bound organelles, are nonetheless highly organized. Specifically, many proteins that are produced in the bacterial cytosoplasm ultimately come to reside in distinct regions of the cell. Efforts over the last decade have revealed that in most of these cases, different proteins are recruited to specific regions of the cell by a pre-localized protein at that location. In this manner, large proteinaceous structures are built in a highly ordered fashion, whereby proteins that comprise it localize by recognizing the protein that arrived just before it. But how does the very first protein get there? We are examining the assembly of the bacterial spore coat, a large structure that is composed of some seventy different proteins, in the model organism Bacillus subtilis. During spore formation, the rod-shaped B. subtilis elaborates a spherical internal organelle that will eventually mature into the spore. Proteins that comprise the spore coat are produced in the cytosol of the outer cell and are deposited onto the surface of this spherical organelle. Among the very first coat proteins that arrive at this surface is called VM. How does VM specifically localize to this surface? The surface of this internal organelle is the only convex, or positively curved surface in the entire cytosol of the bacterium. In my postdoctoral work in the laboratory of Richard Losick at Harvard University (which concluded in August, 2009), we discovered that VM localizes specifically to this surface by recognizing its convex shape, and actively discriminates against concave, or negatively curved, surfaces. Thus, we concluded that the shape of a membrane (in this case, convex curvature) can drive the subcellular localization of a morphogenetic protein. Convex surfaces, however, are fairly rare in bacteria. A more longstanding challenge in bacterial cell biology has been to understand how proteins localize to the poles of rod-shaped bacteria. Could the extreme negative curvature of present at these sites serve as a beacon to recruit some proteins? We discovered that, indeed, the localization of a protein called DivIVA (a protein involved in cell division in B. subtilis) localizes preferentially to highly negatively curved, or concave, membranes. Taken together, we have concluded that recognition of membrane curvature represents a novel protein sorting mechanism. In future work, we will attempt to identify how widespread this protein targeting mechanism may be, and will elucidate the molecular mechanisms underlying the recognition of membrane curvature by proteins.