The role of ITAM- and ITIM-coupled receptors in platelet activation by collagen

The major activation-inducing collagen receptor glycoprotein VI (GPVI) has been cloned within the last two years. It is a member of the Ig superfamily of proteins and is constitutively associated with the ITAM-bearing Fc rece ptor gamma -chain (FcR gamma -chain). GPVI signals through a pathway that i nvolves several of the proteins used by Fc, B- and T-lymphocyte receptors a nd which takes place in glycolipid-enriched membrane domains in the plasma membrane known as GEMs. Responses to GPVI are regulated by PECAM-1 (CD31) a nd possibly other ITIM-bearing receptors. Despite a pivotal role for GPVI, there are important differences between signalling events to collagen and G PVI-specific ligands. This may reflect a role for co-receptors in the respo nse to collagen.