| The localization of proteins to one pole of a rod-shaped bacterium is of particular interest since this is a prerequisite for a number of cellular processes such as cell division, motility and chemotaxis. With few exceptions, the molecular mechanisms that recruit or retain proteins to specific subcellular loci remain to be discovered. The sporulation process of Bacillus subtilis is an especially attractive model system for elucidating mechanisms by which this might occur because it is a genetically amenable organism, sporulation is not essential for viability and during the process, scores of proteins localize to the nascent spore. During sporulation, a rod-shaped cell divides asymmetrically yielding a small compartment (the forespore) and a large compartment (the mother cell); many proteins in each cell localize to the pole that separates these compartments. Here, we have characterized the molecular mechanism that retains SpoIIIAH, an integral membrane protein with a large extracytoplasmic domain, at the asymmetric septum. SpoIIIAH is made in the mother cell and inserted into the membrane at random sites; its subsequent localization requires two events. First, the peptidoglycan that separates the two cells of the sporangium must be hydrolysed. With the dissolution of the septal peptidoglycan, there is no physical barrier precluding physical interaction between SpoIIIAH and proteins extruding from the adjacent cell. In the second step of SpoIIIAH localization, the extracellular domain of a membrane protein made in the forespore, SpoIIQ, binds to SpoIIIAH and retains it in the membranes of the mother cell immediately adjacent the forespore. Employing this mechanism, a protein in the mother cell will only be found in the membranes adjacent the forespore but this approach to localization has broader implications. When SpoIIIAH and SpoIIQ are co-expressed in a cell with an invaginating membrane, they both localize at the site of invagination. This suggests this localization mechanism could be more broadly used by proteins involved in cell division to localize to a nascent septum. |
