Casein kinase Iα (CKIα) belongs to a family of serine/threonine protein kinases involved in membrane trafficking, RNA processing, mitotic spindle formation and cell cycle progression. In this report, we identified several CKIα interacting proteins including RCC1, high mobility group proteins 1 and 2 (HMG1, HMG2), Erf, centaurin-α1, synaptotagmin IX and CPI-17 that were isolated from brain as CKIα co-purifying proteins. Actin, importin-α1, importin-β, PP2Ac, centaurin-α1, and HMG1 were identified by affinity chromatography using a peptide column comprising residues 214–233 of CKIα. We have previously shown that centaurin-α1 represents a CKIα partner both in vitro and in vivo. The nuclear protein regulator of chromosome condensation 1 (RCC1) is a guanosine nucleotide exchange factor for Ran which is involved in nuclear transport and mitotic spindle formation. Here we show that CKIα and RCC1 interact in brain and in cultured cells. However, the interaction does not involve residues 217–233 of CKIα which are proposed from X-ray structures to represent an anchoring site for CKI partners. Formation of the RCC1/CKIα complex is consistent with the association of the kinase with mitotic spindles. In conclusion, we have identified a number of novel CKIα protein partners and their relations to CKI are discussed.
