Differential localization of membrane receptor chemotaxis proteins in the Caulobacter predivisional cell

The methyl-accepting chemotaxis proteins (MCPs) are membrane receptors that initiate signal transduction to the flagellar rotor upon ligand binding. The synthesis of these proteins occurs only in the Caulobacter crescentus predivisional cell coincident with the biosynthesis of the polar flagellum. Both the flagellum and the MCPs are partitioned to only one daughter cell, the swarmer cell, upon division. We report the results of experiments designed to determine the distribution of these MCPs within swarmer cells and predivisional cells. Flagellated and non-flagellated vesicles were prepared from these cells by immunoaffinity chromatography and the level of MCPs that had been labeled either in vivo or in vitro with methyl-3H was determined. Small membrane vesicles from swarmer cells contained [methyl-3H]MCPs both in the flagellated and non-flagellated vesicles, which indicates that the region immediately surrounding the flagellum, as well as the rest of the surface of the swarmer cell, contains [methyl-3H]MCP. Thus, the MCPs are not specifically localized to the immediate vicinity of the flagellar rotor. The distribution of MCPs was examined in flagellated and non-flagellated vesicles isolated from predivisional cells. The analysis of small predivisional vesicles showed that the MCP content is higher in the flagellated vesicles, and analysis of large flagellated vesicles showed that the MCPs are positioned preferentially in the swarmer cell portion of the predivisional cell. This positional bias of MCPs within predivisional cells could reflect either a large compartment or membrane domain within the incipient swarmer cell, or a gradient of MCPs, with the highest concentration in the vicinity of the flagellum.