Ca2+/Calmodulin directly interacts with the pleckstrin homology domain of AKT1

AKT kinase, also known as protein kinase B, is a key regulator of cell growth, proliferation, and metabolism. The activation of the AKT signaling pathway is one of the most frequent molecular alterations in a wide variety of human cancers. Dickson and coworkers recently observed that Ca2(+) center dot calmodulin ( Ca2(+) center dot CaM) may be a common regulator of AKT1 activation ( Deb, T. B., Coticchia, C. M., and Dickson, R. B. ( 2004) J. Biol. Chem. 279, 38903 - 38911). In our efforts to scan the mRNA-displayed proteome libraries for Ca2(+) center dot CaM- binding proteins, we found that both human and Caenorhabditis elegans AKT1 kinases bound to CaM in a Ca2(+)- dependent manner ( Shen, X., Valencia, C. A., Szostak, J., Dong, B., and Liu, R. ( 2005) Proc. Natl. Acad. Sci. U. S. A. 102, 5969 - 5974 and Shen, X., Valencia, C. A., Gao, W., Cotten, S. W., Dong, B., Chen, M., and Liu, R. ( 2007) submitted for publication). Here we demonstrate that Ca2 (+)center dot CaM and human AKT1 were efficiently co-immunoprecipitated, and their interaction was direct rather than mediated by other proteins. The binding is in part attributed to the first 42 residues of the pleckstrin homology ( PH) domain, a region that is critical for the recognition of its lipid ligands. The PH domain of human AKT1 can disrupt the complex of the full-length AKT1 with Ca2 (+) center dot CaM. In addition, Ca2(+) center dot CaM competes with phosphatidylinositol 3,4,5- trisphophate for interaction with the PH domain of human AKT1. Our findings suggest that Ca2(+) center dot CaM is directly involved in regulating the functions of AKT1, presumably by releasing the activated AKT1 from the plasma membrane and/ or prohibiting it from re-association with phosphoinositides on plasma membrane.