The receptor, called latrophilin, is known only as a binding site for the black widow poison latrotoxin. Although no endogenous ligand for latrophilin is known, a varied family of receptors exists with a similar organization. Each family member has two domains: an NH2-terminal fragment (NTF) that interacts with other cell surface or possibly extracellular matrix proteins, and a COOH-terminal fragment (CTF) that resembles a G-protein–coupled receptor (GPCR). The two domains were known to be cleaved, but the persistence of the NTF on the cell surface led previous workers to assume that the transmembrane-less NTF must remain bound to CTF.
The London group now shows that this is not the case. The two fragments have distinct localizations and can be aggregated away from each other using antibodies. Addition of a latrotoxin variant, however, induces clustering of the fragments and signaling.
For the cell, the fusion of two protein functions allows for coordinated expression, but cleavage allows divergent regulation. For example, the NTF may stay attached extracellularly even as the CTF is internalized to allow for GPCR desensitization. Ushkaryov now hopes to test whether different members of the protein family mix and match their respective halves.