Proteolysis of the Caulobacter McpA chemoreceptor is cell cycle regulated by a ClpX-dependent pathway

Proteolysis is involved in cell differentiation and the progression through the cell cycle in Caulobacter crescentus. We have constitutively expressed the transmembrane chemoreceptor McpA from a multicopy plasmid to demonstra te that McpA degradation is modulated during the cell cycle. The level of M cpA protein starts to decrease only when the swarmer cells differentiate in to stalked cells. The reduction in McpA protein levels is maintained until the stalked cells develop into predivisional cells, at which point the leve l returns to that observed in swarmer cells. The cell-cycle-regulated degra dation of McpA does not require the last 12 C-terminal amino acids, but it does require three amino acids (AAL) located 15 residues away from the C te rminus. The ClpXP protease is essential in C. crescentus for viability, and thus, we tested McpA degradation in xylose conditional mutants. The effect on McpA degradation occurred within two generations from the start of ClpX depletion. The conditional mutants' growth rate was only slightly affected , suggesting that ClpX is directly involved in McpA proteolysis.