orphan adhesion receptor GPR64 and related proteins, member of subfamily B2 of the class B secretin-like receptors of seven-transmembrane G protein-coupled receptors
GPR64 is an orphan receptor that has been classified as that belongs to the Group VIII of adhesion GPCRs. Other members of the Group VII include orphan GPCRs such as GPR56, GPR97, GPR112, GPR114, and GPR126. GPR64 is mainly expressed in the epididymis of male reproductive tract, and targeted deletion of GPR64 causes sperm stasis and efferent duct blockage due to abnormal fluid reabsorption, resulting in male infertility. GPR64 is also overexpressed in Ewing's sarcoma (ES), as well as upregulated in other carcinomas from kidney, prostate or lung, and promotes invasiveness and metastasis in ES via the upregulation of placental growth factor (PGF) and matrix metalloproteinase (MMP) 1. The adhesion receptors are characterized by the presence of large N-terminal extracellular domains containing multiple adhesion motifs, which play critical roles in cell-cell adhesion and cell-matrix interactions, that are coupled to a class B seven-transmembrane domain. Furthermore, almost all adhesion receptors, except GPR123, contain an evolutionarily conserved GPCR- autoproteolysis inducing (GAIN) domain that undergoes autoproteolytic processing at the GPCR proteolysis site (GPS) motif located immediately N-terminal to the first transmembrane region, to generate N- and C-terminal fragments (NTF and CTF), which may serve important biological functions.