p57^Kip2 Phosphorylation Modulates Its Localization, Stability, and Interactions

p57^Kip2 is a member of the cyclin-dependent kinase (CDK) Interacting Protein/Kinase Inhibitory Protein (CIP/Kip) family that also includes p21^Cip1/WAF1 and p27^Kip1. Different from its siblings, few data are available about the p57^Kip2 protein, especially in humans. Structurally, p57^Kip2 is an intrinsically unstructured protein, a characteristic that confers functional flexibility with multiple transient interactions influencing the metabolism and roles of the protein. Being an IUP, its localization, stability, and binding to functional partners might be strongly modulated by post-translational modifications, especially phosphorylation. In this work, we investigated by two-dimensional analysis the phosphorylation pattern of p57^Kip2 in different cellular models, revealing how the human protein appears to be extensively phosphorylated, compared to p21^Cip1/WAF1 and p27^Kip1. We further observed clear differences in the phosphoisoforms distributed in the cytosolic and nuclear compartments in asynchronous and synchronized cells. Particularly, the unmodified form is detectable only in the nucleus, while the more acidic forms are present in the cytoplasm. Most importantly, we found that the phosphorylation state of p57^Kip2 influences the binding with some p57^Kip2 partners, such as CDKs, LIMK1 and CRM1. Thus, it is necessary to completely identify the phosphorylated residues of the protein to fully unravel the roles of this CIP/Kip protein, which are still partially identified.