Localization of phosphorylated CK2α to the mitotic spindle requires the peptidyl-prolyl isomerase Pin1

CK2 is a serine/threonine kinase with many substrates, largely unknown modes of regulation and essential roles in mitotic progression. CK2 alpha, a catalytic subunit of CK2, is phosphorylated in mitosis, and here we examine the effect of phosphorylation on CK2 alpha localization. Using phosphospecific antibodies, we show that CK2 alpha localizes to the mitotic spindle in a phosphorylation-dependent manner. Mitotic spindle localization requires the unique C-terminus of CK2 alpha, and involves a novel regulatory mechanism in which phosphorylation of CK2 alpha facilitates binding to the peptidyl-prolyl isomerase Pin1, which is required for CK2. mitotic spindle localization. This could explain how the constitutive activity of CK2 alpha might be targeted towards mitotic substrates. Furthermore, because Pin1 has many important spindle substrates, this might represent a general mechanism for localization of mitotic signalling proteins.