MEMBRANE ASSOCIATION OF LIPOPROTEIN-LIPASE AND A CAMP-BINDING ECTOPROTEIN IN RAT ADIPOCYTES

cAMP-binding ectoprotein (Gce1) and lipoprotein lipase (LPL) are ancho red to plasma membranes of rat adipocytes by glycosylphosphatidylinosi tol (GPI) moieties as demonstrated by cleavage by bacterial phosphatid ylinositol-specific: phospholipase C (PI-PLC), reactivity with anti-cr ossreacting determinant antibodies (anti-CRD), and metabolic labeling with radiolabeled palmitic acid and myoinositol. Quantitative release from the membrane of LPL and Gce1 requires both lipolytic removal of t heir GPI anchors and the presence of either 2 M NaCl or 1 mM inositol 1,2-cyclic monophosphate or inositol 1-monophosphate. PI-PLC-cleaved a nd released LPL or Gce1 reassociates with isolated plasma membranes of rat adipocytes and, less efficiently, with membranes of 3T3 fibroblas ts. The specificity of the reassociation is demonstrated (i) by its in hibition after pretreatment of the membranes with trypsin, (ii) by its competition with inositol 1,2-cyclic monophosphate anti inositol 1-mo nophosphate in a concentration-dependent manner, and (iii) by the limi ted number of binding sites. Enzymic or chemical removal as well as ma sking with anti-CRD antibodies of the terminal inositol (cyclic) monop hosphate moiety of hydrophilic Gce1 and LPL significantly impairs the reassociation. These data suggest that in rat adipocytes GPI-proteins are not readily released from the cell surface upon lipolytic cleavage , but remain associated through a receptor which specifically recogniz es the terminal inositol (cyclic) monophosphate epitope of the (G)PI-P LC-cleaved GPI moiety. This interaction may have implications for the regulated membrane release of GPI-proteins and for their possible inte rnalization.