Histidine kinase, Adenylyl cyclase, Methyl-accepting protein, and Phosphatase (HAMP) domain. HAMP is a signaling domain which occurs in a wide variety of signaling proteins, many of which are bacterial. The HAMP domain consists of two alpha helices connected by an extended linker. The structure of the HAMP dimer from Archaeoglobus fulgidus has been solved using nuclear magnetic resonance, revealing a parallel four-helix bundle; this structure has been confirmed by cross-linking analysis of HAMP domains from the Escherichia coli aerotaxis receptor Aer. It has been suggested that the four-helix arrangement can rotate between the unusually packed conformation observed in the NMR structure and a canonical coiled-coil arrangement. Such rotation may coincide with signal transduction, but a common mechanism by which HAMP domains relay a variety of input signals has yet to be established.