Identification and characterization of CKIP-1, a novel pleckstrin homology domain-containing protein that interacts with protein kinase CK2

The catalytic subunits of protein kinase CK2, CK2 alpha and CK2 alpha', are closely related to each other but exhibit functional specialization, To test the hypothesis that specific functions of CK2 alpha and CK2 alpha' are mediated by specific interaction partners, we used the yeast two-hybrid system to identify CK2 alpha- or CK2 alpha'-binding proteins. We report the identification and characterization of a novel CK2-interacting protein, designated CKIP-1, that interacts with CK2 alpha, but not CK2 alpha', in the yeast two-hybrid system. CKIP-1 also interacts with CK2 alpha in vitro and is co-immunoprecipitated from cell extracts with epitope-tagged CK2 alpha and an enhanced green fluorescent protein fusion protein encoding CKIP-1 (i.e. EGFP-CKIP-1) when they are co-expressed. CK2 activity is detected in anti-CKIP-1 immunoprecipitates performed with extracts from non-transfected cells indicating that CKIP-1 and CK2 interact under physiological conditions, The CKIP-1 cDNA is broadly expressed and encodes a protein with a predicted molecular weight of 46,000, EGFP-CKIP-1 is localized within the nucleus and at the plasma membrane. The plasma membrane localization is dependent on the presence of an amino-terminal pleckstrin homology domain. We postulate that CKIP-1 is a non-enzymatic regulator of one isoform of CK2 (i.e. CK2 alpha) with a potential role in targeting CK2 alpha to a particular cellular location.