MUTATION-DELETION ANALYSIS OF A CA2+-DEPENDENT PHOSPHOLIPID-BINDING (CALB) DOMAIN WITHIN P120 GAP, A GTPASE-ACTIVATING PROTEIN FOR P21 RAS

p120 GAP is a GTPase activating protein for p21 ras. It is a multidomain protein which exhibits sequence similarity with other GTPase-activating proteins, src, pleckstrin and a central portion of the protein kinase C conserved region 2 domain known as CaLB (Ca2+-dependent phospholipid-binding). The presence of this CaLB motif has led to the speculation that p120 GAP may be a member of a family of structurally related proteins containing a Ca2+-dependent membrane/lipid-binding domain. Here we have studied the in vitro Ca2+-dependent phospholipid-binding properties of the isolated proposed CaLB sequence in human GAP and deduce that a phospholipid-binding sequence is indeed located between amino acids 606 and 648. Binding of phosphatidylserine and phosphatidylinositol, but not phosphatidylcholine, within this sequence is Ca2+-dependent, with an estimated EC(50) for Ca2+ of approx. 1 mu M. Using deletion-mutation analysis we have further defined the minimal boundaries for this in vitro phospholipid-binding activity, p120 GAP amino acids 612-643 exhibit full phospholipid-binding activity, but further deletion of either amino acids 612-617 or amino acids 633-648 significantly decreased or abolished phospholipid binding. These studies establish that amino acids 612-643 of p120 GAP indeed constitute a functional CaLB domain and thereby imply a role for Ca2+ in the regulation of p120 GAP association with cellular (membrane) phospholipids.