Identification of the general transcription factor Yin Yang 1 as a novel and specific binding partner for S6 kinase 2.

S6 kinase is a member of the AGC family of serine/threonine kinases and plays a key role in diverse cellular processes including cell growth and metabolism. Although, the high degree of homology between S6K family members (S6K1 and S6K2) in kinase and kinase-extension domains, the two proteins are highly divergent in the N- and C-terminal regulatory regions, hinting at differential regulation, downstream signalling and cellular function. Deregulated signalling via S6Ks has been linked to various human pathologies, such as diabetes and cancer. Therefore, S6K has emerged as a promising target for drug development. Much of what we know about S6K signalling in health and disease comes from studies of S6K1, as molecular cloning of this isoform was reported a decade earlier than S6K2. In this study, we report for the first time, the identification of the general transcription factor Yin Yang 1 (YY1) as a novel and specific binding partner of S6K2, but not S6K1. The interaction between YY1 and S6K2 was demonstrated by co-immunoprecipitation of transiently overexpressed and endogenous proteins in a number of cell lines, including HEK293, MCF7 and U937. Furthermore, direct association between S6K2 and YY1 was demonstrated by GST pull-down assay using recombinant proteins. A panel of deletion mutants was used to show that the C-terminal regulatory region of S6K2 mediates the interaction with YY1. Interestingly, the complex formation between S6K2 and YY1 can be detected in serum-starved cells, but the interaction is strongly induced in response to mitogenic stimulation. The induction of S6K2/YY1 complex formation in response to serum stimulation is abolished by pre-treatment of cells with the mTOR inhibitor, rapamycin. Furthermore, mTOR is also detected in complex with YY1 and S6K2 in serum-stimulated cells. We utilized size exclusion chromatography along with co-immunoprecipitation analysis to demonstrate the existence of the mTOR/S6K2/YY1 complex in high molecular weight fractions, which might also involve other cellular proteins. The physiological significance of the mTOR/S6K2/YY1 complex, which is induced in response to mitogenic stimulation, remains to be further investigated.