Phosphatase to kinase switch of a critical enzyme contributes to timing of cell differentiation

Abstract

ABSTRACT Cell differentiation is an essential biological process that is subject to strict temporal regulation. Caulobacter crescentus undergoes obligate differentiation from a swarmer cell to a stationary, replication-competent stalked cell with each cell cycle. We report that the switch from phosphatase to kinase activity of the histidine kinase PleC contributes to timing this differentiation event. PleC P er- A rnt- S im (PAS) domain interaction with the polar scaffold protein PodJ localizes PleC to the cell pole and inhibits in vivo kinase activity. Upon PodJ degradation, released PleC switches to its kinase form and phosphorylates the PleD diguanylate cyclase, initiating the signaling pathway responsible for differentiation. While PodJ inhibits PleC kinase activity, it does not impact PleC phosphatase activity on DivK, which is required for pili biogenesis and flagellar rotation. Thus, PleC PAS domains affect enzymatic function on diverse substrates by relying on context-dependent binding partners, thereby controlling the timing of Caulobacter cell differentiation. IMPORTANCE The process of cell differentiation is highly regulated in both prokaryotic and eukaryotic organisms. The aquatic bacterium, Caulobacter crescentus , undergoes programmed cell differentiation from a motile swarmer cell to a stationary stalked cell with each cell cycle. This critical event is regulated at multiple levels. Kinase activity of the bifunctional enzyme, PleC, is limited to a brief period when it initiates the molecular signaling cascade that results in cell differentiation. Conversely, PleC phosphatase activity is required for pili formation and flagellar rotation. We show that PleC is localized to the flagellar pole by the scaffold protein, PodJ, which is known to suppress PleC kinase activity in vitro . PleC mutants that are unable to bind PodJ have increased kinase activity in vivo , resulting in premature differentiation. We propose a model in which PodJ regulation of PleC’s enzymatic activity contributes to the robust timing of cell differentiation during the Caulobacter cell cycle.