Calmodulin binds to p21(Cip1) and is involved in the regulation of its nuclear localization

p21(Cip1), first described as an inhibitor of cyclin-dependent kinases, has recently been shown to have a function in the formation of cyclin D-Cdk4 c omplexes and in their nuclear translocation. The dual behavior of p21(Cip1) may be due to its association with other proteins. Different evidence pres ented here indicate an in vitro and in vivo interaction of p21(Cip1) With c almodulin: 1) purified p21(Cip1) is able to bind to calmodulin-Sepharose in a Ca2+-dependent manner, and this binding is inhibited by the calmodulin-b inding domain of calmodulin-dependent kinase II; 2) both molecules coimmuno precipitate when extracted from cellular lysates; and 3) colocalization of calmodulin and p21(Cip1) can be detected in vivo by electron microscopy imm unogold analysis. The carboxyl-terminal domain of p21(Cip1) is responsible for the calmodulin interaction, since p21(145-164) peptide is also able to bind calmodulin and to compete with full-length p21(Cip1) for the calmoduli n binding. Because treatment of cells with anti-calmodulin drugs decreases the nuclear accumulation of p21(Cip1), We hypothesize that calmodulin inter action with p21(Cip1) is important for p21(Cip1), and in consequence for cy clin D-Cdk4, translocation into the cell nucleus.