Abstract

Casein kinase II (CKII) is a protein serine/threonine kinase known to control the activity of a variety of regulatory nuclear proteins. This enzyme has a tetrameric structure composed of two catalytic (alpha and/or alpha ') subunits and two beta subunits. We have examined the subunit composition of tetrameric complexes of purified bovine CKII by immunoprecipitation using alpha, alpha ', or beta subunit-specific antibodies. These experiments indicate that the enzyme can exist as homotetramers (i.e., alpha 2 beta 2 or alpha 2' beta 2) as well as heterotetramers (i.e. alpha alpha ' beta 2). To further examine subunit interactions between the alpha, alpha ', or beta subunits of CKII, we have utilized the yeast two-hybrid system (Fields, S. and Song, O. (1989) Nature 340: 245-246). For these studies, each subunit of human CKII was expressed in yeast as a fusion with the DNA binding domain or with the transcriptional activation domain of the yeast GAL4 transcriptional activator. These studies demonstrate that the alpha or alpha ' subunits of CKII can interact with the beta subunits of CKII, but not with other alpha or alpha ' subunits. By comparison, the beta subunits of CKII can interact with alpha, alpha ', or beta subunits. These results indicate that the CKII holoenzyme forms because of the ability of beta subunits to dimerize, bringing two heterodimers (alpha beta or alpha ' beta) into a tetrameric complex.