PIG-tailed membrane proteins.

Some membrane proteins are associated with the plasma membrane solely through a glycolipid moiety (GPI anchor). The GPI anchor is composed of a core structure of phosphatidylinositol attached to a glycan chain which, in turn, is attached to the C-terminus of the protein. The GPI-anchored protein can be released from the cell surface by the action of GPI-specific phospholipases C and D. In protozoa, GPI anchors represent the predominant mechanism for integrating cell-surface proteins into the lipid bilayer. Addition of a glycolipid anchor to a nascent protein requires a C-terminal hydrophobic signal sequence on the protein which is rapidly exchanged for a pre-assembled anchor. GPI anchors may have roles in protein targeting, cell signalling and in the uptake of small molecules (potocytosis). The human disease 'paroxysmal nocturnal haemoglobinuria' represents a defect in biosynthesis of the GPI anchor. Other lipid post-translational modifications of proteins are also recognized as important in regulating protein function (myristoylation, palmitoylation, prenylation).