Recently a novel mechanism has been described for the hydrophobic atta
chment of proteins to membranes which is shared by membrane proteins o
f widely differing origins and functions. The hydrophobic anchor is a
glycosyl-phosphatidylinositol (GPI) which is covalently attached to th
e polypeptide chain. GPI-anchored proteins have been reported to resid
e in clusters collected over small membrane invaginations called caveo
lae. GPI-anchored proteins constitute an exceptionally diverse family
of membrane molecules functioning in such processes as nutrient uptake
, cell adhesion, catalysis, and membrane signalling events. A growing
body of biochemical evidence indicates that they play a key role in th
e recently discovered potocytosis process, act as intracellular sortin
g signals, cause activation of Src-family kinase-mediated signalling p
athways. Their hydrolysis, yielding inositolphosphoglycans and diacylg
lycerol, is one of the earliest intracellular events generated in resp
onse to growth factors, insulin or lipoproteins.