Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous c lass of eukaryotic glycolipids. Although these structures were originally d iscovered in the form of GPI-anchored cell surface glycoproteins, it is bec oming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of no nprotein-linked GPIs can approach 10(7) molecules per cell in some cell typ es, especially the protozoa, with a large proportion of these molecules bei ng displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between spe cies and cell types, (b) complexity in the topological arrangement of the G PI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restr iction of the biosynthetic pathway within the endoplasmic reticulum. Furthe rmore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic path way appear to be widely distributed in the cellular endomembrane system. Th ese studies indicate that there is still much to learn about the organizati on of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the f unction of these lipids within cells.