α-Latrotoxin Receptor CIRL/Latrophilin 1 (CL1) Defines an Unusual Family of Ubiquitous G-protein-linked Receptors

α-Latrotoxin, a potent excitatory neurotoxin, binds to two receptors: a G-protein-coupled receptor calledCIRL/latrophilin 1 (CL1) and a cell-surface protein called neurexin Iα. We now show that CL1 belongs to a family of closely related receptors called CL1, CL2, and CL3. CLs exhibit an unusual multidomain structure with similar alternative splicing and large extra- and intracellular sequences. CLs share domains with other G-protein-coupled receptors, lectins, and olfactomedins/myocilin. In addition, CLs contain 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 tissues. To examine how binding of α-latrotoxin to CL1 triggers exocytosis, we used PC12 cells transfected with human growth hormone. Ca2+-dependent secretion of human growth hormone from transfected PC12 cells was triggered by KCl depolarization or α-latrotoxin and was inhibited by tetanus toxin and by phenylarsine oxide, a phosphoinositide kinase inhibitor. When CL1 was transfected into PC12 cells, their response to α-latrotoxin was sensitized dramatically. A similar sensitization to α-latrotoxin was observed 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 transmembrane region and presumably is unable to mediate G-protein-signaling was as active as wild type CL1 in α-latrotoxin-triggered exocytosis. Our data 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 α-latrotoxin receptor that does not require G-protein-signaling for triggering exocytosis. This suggests that as an α-latrotoxin receptor, CL1 recruits α-latrotoxin to target membranes without participating in exocytosis directly.