N-terminal protein-binding module of harmonin and similar domains, also known as HHD (harmonin homology domain)
This domain is found in harmonin, and similar proteins such as delphilin, and whirlin. These are postsynaptic density-95/discs-large/ZO-1 (PDZ) domain-containing scaffold proteins. Harmonin and whirlin are organizers of the Usher protein network of the inner ear and the retina, delphilin is found at the cerebellar parallel fiber-Purkinje cell synapses. This domain is also found in CCM2 (also called malcavernin; C7orf22/chromosome 7 open reading frame 22; OSM). CCM2 along with CCM1 and CCM3 constitutes a set of proteins which when mutated are responsible for cerebral cavernous malformations, an autosomal dominant neurovascular disease characterized by cerebral hemorrhages and vascular malformations in the central nervous system. CCM2 plays many functional roles. CCM2 functions as a scaffold involved in small GTPase Rac-dependent p38 mitogen-activated protein kinase (MAPK) activation when the cell is under hyperosmotic stress. It associates with CCM1 in the signaling cascades that regulate vascular integrity and participates in HEG1 (the transmembrane receptor heart of glass 1) mediated endothelial cell junctions. CCM proteins also inhibit the activation of small GTPase RhoA and its downstream effector Rho kinase (ROCK) to limit vascular permeability. CCM2 mediates TrkA-dependent cell death via its N-terminal PTB domain in pediatric neuroblastic tumours; the C-terminal domain of malcavernin represented here has also been refered to as the Karet domain. Harmonin contains a single copy of this domain at its N-terminus which binds specifically to a short internal peptide fragment of the cadherin 23 cytoplasmic domain (a component of the Usher protein network). Whirlin contains two copies of this domain; the first of these has been assayed for interaction with the cytoplasmic domain of cadherin 23 and no interaction could be detected.
Jiyao W et al.(2023). "The conserved domain database in 2023.",