The Ca super(2+)-dependent lipid binding domain of P120 super(GAP) mediates protein-protein interactions with Ca super(2+)-dependent membrane-binding proteins. Evidence for a direct interaction between annexin VI and P120 super(GAP)

The CaLB domain is a 43-amino acid sequence motif found in a number of functionally diverse signaling proteins including three Ras-specific GTPase activating proteins (GAPs). In the Ras GTPase activating protein, P120 super(GAP), this domain has the ability to confer membrane association in response to intracellular Ca super(2+) elevation. Here we have isolated three proteins, p55, p70, and p120, which interact with the P120 super(GAP) CaLB domain in vitro. We identify p70 as the Ca super(2+)-dependent phospholipid-binding protein annexin VI. Using co-immunoprecipitation studies, we have shown that the interaction between P120 super(GAP) and annexin VI is also detectable in rat fibroblasts, suggesting that this interaction may have a physiological role in vivo. Thus, the CaLB domain in P120 super(GAP) appears to have the ability to direct specific protein-protein interactions with Ca super(2+)-dependent membrane-associated proteins. In addition, annexin VI is known to have tumor suppressor activity. Therefore, it is possible that the interaction of annexin VI with P120 super(GAP) may be important in the subsequent modulation of p21 super(ras) activity.