Functional specialization of CK2 isoforms and characterization of isoform-specific binding partners

In mammals, protein kinase CK2 has two isozymic forms of its catalytic subu nit, designated CK2 alpha and CK2 alpha'. CK2 alpha and CK2 alpha' exhibit extensive similarity within their catalytic domains but have completely unr elated C-terminal sequences. To systematically examine the cellular functio ns of each CK2 isoform in mammalian cells, we have generated human osteosar coma U2-OS cell lines with the expression of active or inactive versions of each CK2 isoform under the control of an inducible promoter [22]. Examinat ion of these cell lines provides evidence for functional specialization of CK2 isoforms at the cellular level in mammals with indications that CK2 alp ha' is involved in the control of proliferation and/or cell survival. To un derstand the molecular basis for functional differences between CK2 alpha a nd CK2 alpha', we have undertaken studies to identify proteins that interac t specifically with each isoform of CK2 and could contribute to the regulat ion of their independent functions. A novel pleckstrin-homology domain cont aining protein, designated CK2-interacting protein 1 (i.e. CKIP-1) was isol ated using the yeast two hybrid system as a protein that interacts with CK2 alpha but not CK2 alpha' [23]. When expressed in cells as a fusion with gr een fluorescent protein, CKIP-1 localizes to the cell membrane and to the n ucleus. In this study, we present evidence from deletion analysis of CKIP-1 suggesting that a C-terminal region containing a putative leucine zipper h as a role in regulating its nuclear localization. Collectively, our data su pports a model whereby CKIP-1 is a non-enzymatic regulator of CK2 alpha tha t regulates the cellular functions of CK2 alpha by targeting or anchoring C K2 alpha to specific cellular localization or by functioning as an adapter to integrate CK2 alpha -mediated signaling events with components of other signal transduction pathways.