Plays an essential role in chemotaxis. Regulates flagellar rotation through the formation of a complex with chemotaxis protein CheZ (PubMed:21791582, PubMed:26061894). Plays a major role in colonization of the stomach (PubMed:21791582). Interacts with CheZ; the interaction is essential for each other polar localization. Localizes to the cell poles. Deletion mutants cannot control the rotation of their flagella and swim with abnormally frequent reversals.