The K protein was first identified in the heterogeneous ribonucleoprotein particle (hnRNP). Subsequently, K protein was shown to bind sequence-specific single-and double-stranded DNA, stimulate transcription, and bind Src, Fyn, Lyn, and Vav via SH3 interactions. The K protein also binds to the kappa B enhancer motif which stimulates its phosphorylation in vitro by an associated serine/threonine kinase. To gain more insight into this unique nucleic acid-dependent phosphorylation process, we set out to examine the regulation of this kinase. We demonstrate that the K protein exists in a complex with an IL-1-responsive kinase and that phosphorylation of the K protein by this kinase is augmented by either cognate DNA or RNA sequences. The IL-1-responsive kinase activity associated with the K protein is reduced by phosphatase treatment, suggesting that the K protein kinase activity is regulated by phosphorylation. The observation that phosphorylation of the K protein is DNA- or RNA-dependent and IL-1-responsive suggests that the function of the K protein is tightly regulated.