The highly conserved domain of the Caulobacter McpA chemoreceptor is required for its polar localization

We have fused GFP to the C-terminus of McpA to study chemoreceptor polar lo calization in Caulobacter crescentus. The full-length McpA-GFP fusion is po larly localized and methylated. The methylation is dependent on the chemore ceptor methyltransferase (cheR) and chemoreceptor methylesterase (cheB) gen es present in the mcpA operon. C-terminal and internal deletions of McpA we re constructed and fused to the N-terminus of GFP to identify the domains r equired for polar localization. When the R1 methylation domain was deleted, the McpA-GFP fusion was still polarly localized, suggesting that this doma in is dispensable for polar localization. However, when the highly conserve d domain (HCD), which is involved in interacting with CheW, was deleted eit her by an internal deletion or C-terminal deletion, the resulting McpA-GFP fusions were completely delocalized. When the mcpA operon, which contains t he cheW and cheA homologues, was deleted, the full-length McpA-GFP fusion w as delocalized. Although additional chemotaxis genes are required for the p olar localization of McpA-GFP, the presence of the single polar flagellum i s not required. However, in filamentous cells, which are frequently found i n C. crescentus fliF mutants, the McpA-GFP fusion was observed at mid-cell positions.