Cell cycle‐controlled proteolysis of a flagellar motor protein that is asymmetrically distributed in the Caulobacter predivisional cell.

Flagellar biogenesis and release are developmental events tightly coupled to the cell cycle of Caulobacter crescentus. A single flagellum is assembled at the swarmer pole of the predivisional cell and is released later in the cell cycle. Here we show that the MS‐ring monomer FliF, a central motor component that anchors the flagellum in the cell membrane, is synthesized only in the predivisional cell and is integrated into the membrane at the incipient swarmer cell pole, where it initiates flagellar assembly. FliF is proteolytically turned over during swarmer‐to‐stalked cell differentiation, coinciding with the loss of the flagellum, suggesting that its degradation is coupled to flagellar release. The membrane topology of FliF was determined and a region of the cytoplasmic C‐terminal domain was shown to be required for the interaction with a component of the motor switch. The very C‐terminal end of FliF contains a turnover determinant, required for the cell cycle‐dependent degradation of the MS‐ring. The cell cycle‐dependent proteolysis of FliF and the targeting of FliF to the swarmer pole together contribute to the asymmetric localization of the MS‐ring in the predivisional cell.