Covalent linkage of proteins to glycosyl-phosphatidylinositol (GPI) molecules is now recognized as an important mechanism for anchoring proteins to membranes. Recent structural work on the GPI anchors indicates that the structure of the glycan connecting the protein and the phosphatidylinositol molecule has been conserved during evolution. Attachment of the protein to the GPI molecule is directed by a signal sequence at the COOH terminus of the polypeptide that is removed during the attachment process. Alternative processing of the same RNA transcript may lead to mRNA species encoding for the same protein but not utilizing GPI for membrane anchoring. One function of the GPI anchor may be to offer a site for degradation by specific endogenous phospholipases with release of the protein from the cell surface. The products of GPI anchor degradation may also have biological activity and be involved in cell communication. Although the physiological role of these events is not certain, the available evidence sug...