A membrane metalloprotease participates in the sequential degradation of a Caulobacter polarity determinant

Abstract

Caulobacter crescentus assembles many of its cellular machines at distinct times and locations during the cell cycle. PodJ provides the spatial cues for the biogenesis of several polar organelles, including the pili, adhesive holdfast and chemotactic apparatus, by recruiting structural and regulatory proteins, such as CpaE and PleC, to a specific cell pole. PodJ is a protein with a single transmembrane domain that exists in two forms, full-length (PodJ_L) and truncated (PodJ_S), each appearing during a specific time period of the cell cycle to control different aspects of polar organelle development. PodJ_L is synthesized in the early predivisional cell and is later proteolytically converted to PodJ_S. During the swarmer-to-stalked transition, PodJ_S must be degraded to preserve asymmetry in the next cell cycle. We found that MmpA facilitates the degradation of PodJ_S. MmpA belongs to the site-2 protease (S2P) family of membrane-embedded zinc metalloproteases, which includes SpoIVFB and YluC of Bacillus subtilis and YaeL of Escherichia coli. MmpA appears to cleave within or near the transmembrane segment of PodJ_S, releasing it into the cytoplasm for complete proteolysis. While PodJ_S has a specific temporal and spatial address, MmpA is present throughout the cell cycle; furthermore, periplasmic fusion to mRFP1 suggested that MmpA is uniformly distributed around the cell. We also determined that mmpA and yaeL can complement each other in C. crescentus and E. coli, indicating functional conservation. Thus, the sequential degradation of PodJ appears to involve regulated intramembrane proteolysis (Rip) by MmpA.