The CHASE domain is an extracellular domain of 200-230 amino acids, which is found in transmembrane receptors from bacteria, lower eukaryotes and plants. It has been named CHASE (Cyclases/Histidine kinases Associated Sensory Extracellular) because of its presence in diverse receptor-like proteins with histidine kinase and nucleotide cyclase domains. The CHASE domain always occurs N-terminally in extracellular or periplasmic locations, followed by an intracellular tail housing diverse enzymatic signalling domains such as histidine kinase (INTERPRO:IPR005467), adenyl cyclase, GGDEF-type nucleotide cyclase and EAL-type phosphodiesterase domains, as well as non-enzymatic domains such PAS (INTERPRO:IPR000014), GAF (INTERPRO:IPR003018), phosphohistidine and response regulatory domains. The CHASE domain is predicted to bind diverse low molecular weight ligands, such as the cytokinin-like adenine derivatives or peptides, and mediate signal transduction through the respective receptors PMID:11590001,PMID:11590000. It is predicted to be a ligand binding domain.

The CHASE domain has a predicted alpha+beta fold, with two extended alpha helices on both boundaries and two central alpha helices separated by beta sheets. The termini are less conserved compared with the central part of the domain, which shows strongly conserved motifs.

The N-terminus of the AHK4 sensor module folds into a long stalk helix followed by two PAS-like domains which are connected by a helical linker. The last beta-strand of the membrane-proximal PAS domain is covalently linked to the N-terminus of the stalk helix by a disulphide bridge, bringing the flanking membrane helices into close proximity PMID:21964459.

INTERPRO:IPR006189,DOI:10.1038/nchembio.667