Cell surface beta 1,4-galactosyltransferase-I activates G protein-dependent exocytotic signaling

ZP3 is a protein in the mammalian egg coat (zona pellucida) that binds sper m and stimulates acrosomal exocytosis, enabling sperm to penetrate the zona pellucida, The nature of the ZP3 receptor/s on sperm is a matter of consid erable debate, but most evidence suggests that ZP3 binds to beta1,4-galacto syltransferase-I (GaITase) on the sperm surface. It has been suggested that ZP3 induces the acrosome reaction by crosslinking GalTase, activating a he terotrimeric G protein. In this regard, acrosomal exocytosis is sensitive t o pertussis toxin and the GalTase cytoplasmic domain can precipitate G(i) f rom sperm lysates, Sperm from mice that overexpress GalTase bind more solub le ZP3 and show accelerated G protein activation, whereas sperm from mice w ith a targeted deletion in GalTase have markedly less ability to bind solub le ZP3, undergo the ZP3-induced acrosome reaction, and penetrate the zona p ellucida. We have examined the ability of GalTase to function as a ZP3 receptor and t o activate heterotrimeric G proteins using Xenopus laevis oocytes as a hete rologous expression system. Oocytes that express GalTase bound ZP3 but did not bind other zona pellucida glycoproteins, After oocyte maturation, ZP3 o r GalTase antibodies were able to trigger cortical granule exocytosis and a ctivation of GalTase-expressing eggs. Pertussis toxin inhibited GalTase-ind uced egg activation. Consistent with G protein activation, both ZP3 and ant i-GalTase antibodies increased GTP gamma[S-35] binding as well as GTPase ac tivity in membranes from eggs expressing GalTase, Finally, mutagenesis of a putative G protein activation moth within the GalTase cytoplasmic domain e liminated G protein activation in response to ZP3 or anti-GalTase antibodie s. These results demonstrate directly that GalTase functions as a ZP3 recep tor and following aggregation, is capable of activating pertussis toxin-sen sitive G proteins leading to exocytosis.