Receptor‐mediated protein kinase activation and the mechanism of transmembrane signaling in bacterial chemotaxis

Chemotaxis responses of Escherichia coli and Salmonella are mediated by type I membrane receptors with N‐terminal extracytoplasmic sensing domains connected by transmembrane helices to C‐terminal signaling domains in the cytoplasm. Receptor signaling involves regulation of an associated protein kinase, CheA. Here we show that kinase activation by a soluble signaling domain construct involves the formation of a large complex, with ∼14 receptor signaling domains per CheA dimer. Electron microscopic examination of these active complexes indicates a well defined bundle composed of numerous receptor filaments. Our findings suggest a mechanism for transmembrane signaling whereby stimulus‐induced changes in lateral packing interactions within an array of receptor‐sensing domains at the cell surface perturb an equilibrium between active and inactive receptor–kinase complexes within the cytoplasm.