ABSTRACT
Phospholipids are essential components of the cell membranes. They were usually considered as relatively inert substances, but in the past few years it has been recognized that inositol phospholipids (phosphoinositides) have an essential role in the transductional mechanisms related to the action of hormones, growth factors, and other chemical messengers involved in signal transduction across the cell membrane. 1-10 The transductional mechanism represented by phosphoinositide metabolism may interact with mechanisms involving tyrosine phosphorylation of cellular proteins. 11 It may also interact with signaling pathways involving the generation of cAMP. 12 The general biological importance of phosphoinositides is suggested by the fact that they are present in vertebrates such as Xenopus laevis, 13 as well as in invertebrates. In the amoebal organism Dictyostelium discoideum, two pathways of signal transduction have been found to operate, one involving adenylate cyclase for signal relay, and the other involving inositol phosphates. 14 Phosphoinositides are present even in plants, 15-18 where they are involved in the transduction of signals across membranes. 19 In addition to their role in transduction mechanisms, phosphoinositides are involved in other cellular functions. Many proteins in both prokaryotic and eukaryotic membranes contain a covalently attached lipid and in some of these proteins the lipid moiety consists of glycosylated phosphatidylinositol, which may be responsible for membrane anchoring. 20
