The role of polar localization in the function of an essential Caulobacter crescentus tyrosine kinase.

DivL is an essential tyrosine kinase in Caulobacter crescentus that controls an early step in the cell division cycle. We show here that DivL dynamically localizes to the stalk-distal cell pole and less frequently to the stalked cell pole during the S-phase. The kinase is subsequently released from the cell poles late in division and remains dispersed in the newly divided progeny stalk and swarmer cells. Mutational analysis of DivL in a DivL-GFP fusion protein demonstrated that the extreme C-terminus and residues in the conserved four-helix bundle, which is the phosphorylation-dimerization domain, are important for localization. We speculate that the four-helix bundle of the core catalytic domain may serve as a recognition site for the "localization machinery". Unexpectedly, a DivL protein with mutations in the C-terminal localization sequence, and an intact catalytic domain, efficiently complemented a divL null mutation. Thus, subcellular localization of DivL is not essential to its function in cell division regulation. Regulation of cell division by DivL does, however, depend on its localization in the cell membrane.