The response regulators for CheA bind to the P2 domain, which is found between Pfam:PF01627 and Pfam:PF02895 as either one or two copies. Highly flexible linkers connect P2 to the rest of CheA and impart remarkable mobility to the P2 domain. This feature is thought to enhance the inter CheA dimer phosphotransfer reactions within the signalling complex, thereby amplifying the phosphorylation signal [1].

1: Histidine kinases: diversity of domain organization. Dutta R, Qin L, Inouye M; Mol Microbiol 1999;34:633-640. PMID:10564504