Calmodulin Binds to p21Cip1 and Is Involved in the Regulation of Its Nuclear Localization

p21Cip1, first described as an inhibitor of cyclin-dependent kinases, has recently been shown to have a function in the formation of cyclin D-Cdk4 complexes and in their nuclear translocation. The dual behavior of p21Cip1 may be due to its association with other proteins. Different evidence presented here indicate an in vitro and in vivo interaction of p21Cip1 with calmodulin: 1) purified p21Cip1 is able to bind to calmodulin-Sepharose in a Ca2+-dependent manner, and this binding is inhibited by the calmodulin-binding domain of calmodulin-dependent kinase II; 2) both molecules coimmunoprecipitate when extracted from cellular lysates; and 3) colocalization of calmodulin and p21Cip1 can be detected in vivo by electron microscopy immunogold analysis. The carboxyl-terminal domain of p21Cip1 is responsible for the calmodulin interaction, since p21145–164 peptide is also able to bind calmodulin and to compete with full-length p21Cip1 for the calmodulin binding. Because treatment of cells with anti-calmodulin drugs decreases the nuclear accumulation of p21Cip1, we hypothesize that calmodulin interaction with p21Cip1 is important for p21Cip1, and in consequence for cyclin D-Cdk4, translocation into the cell nucleus.