alpha-latrotoxin receptor CIRL/latrophilin 1 (CL1) defines an unusual family of ubiquitous G-protein-linked receptors - G-protein coupling not required for triggering exocytosis

alpha-Latrotoxin, a potent excitatory neurotoxin, binds to two receptors: a G-protein-coupled receptor called CIRL/latrophilin 1 (CL1) and a cell-surf ace protein called neurexin I alpha. We now show that CL1 belongs to a fami ly of closely related receptors called CL1, CL2, and CL3, CLs exhibit an un usual multidomain structure with similar alternative splicing and large ext ra- and intracellular sequences, CLs share domains with other G-protein-cou pled receptors, lectins, and olfactomedins/myocilin. In addition, CLs conta in a novel, widespread cysteine-rich domain that may direct endoproteolytic processing of CLs during transport to the cell surface. Although the mRNAs for CLs are enriched in brain, CLs are ubiquitously expressed in all tissu es. To examine how binding of alpha-latrotoxin to CL1 triggers exocytosis, we used PC12 cells transfected with human growth hormone. Ca2+-dependent se cretion of human growth hormone from transfected PC12 cells was triggered b y KCl depolarization or alpha-latrotoxin and was inhibited by tetanus toxin and by phenylarsine oxide, a phosphoinositide kinase inhibitor. When CL1 w as transfected into PC12 cells, their response to alpha-latrotoxin was sens itized dramatically. A similar sensitization to alpha-latrotoxin was observ ed with different splice variants of CL1, whereas CL2 and CL3 were inactive in this assay. A truncated form of CL1 that contains only a single transme mbrane region and presumably is unable to mediate G-protein-signaling was a s active as wild type CL1 in alpha-latrotoxin-triggered exocytosis. Our dat a show that CL1, CL2, and CL3 perform a general and ubiquitous function as G-protein-coupled receptors in cellular signaling. In addition, CL1 serves a specialized role as an alpha-latrotoxin receptor that does not require G- protein-signaling for triggering exocytosis, This suggests that as an alpha -latrotoxin receptor, CL1 recruits alpha-latrotoxin to target membranes wit hout participating in exocytosis directly.