Signalling and non-caveolar rafts

Rafts are small membrane domains containing discrete subsets of lipids and proteins. Although microscopic raft structures termed 'caveolae' were descr ibed nearly 50 years ago, the importance of rafts, particularly signalling within rafts, is only beginning to be understood. Our studies focus on rece ptor-dependent phosphoinositide signalling. Using their characteristic buoy ancy in density gradients, we and others found that the epidermal growth fa ctor (EGF) receptor, phosphatidylinositol 4-kinase and phosphoinositides ar e localized within a caveolin-rich fraction of A431 carcinoma cells. We sub sequently found that membrane fragments containing the EGF receptor and mos t cellular phosphoinositides can be separated from caveolae. Consequently, components of EGF-dependent phosphoinositide signalling localize to one or more novel types of raft, the composition of which we are currently determi ning. A key component is the type II phosphatidylinositol 4-kinase, which, for many years, has proven difficult to purify and clone. We describe our r ecent purification from rafts and cloning of this elusive enzyme, and discu ss how the structure sheds light on the rafting of this enzyme.