Hormonally inducible phosphorylation of a nuclear pool of ribosomal protein S6

Thyrotropin releasing hormone (TRH) and epidermal growth factor induce the rapid phosphorylation of a basic, chromatin-associated protein present in GH4 rat pituitary cells and also found in primary hepatocyte culture. Cell fraction experiments indicate a nucleolar localization for this basic, chromatin-associated protein. The protein has been purified to homogeneity from rat liver and 23 amino acids of its N-terminal sequence determined. There is complete homology between the sequenced portion of the basic, chromatin-associated protein and the N-terminal sequence of rat ribosomal protein S6. In vivo and in vitro phosphorylation, two-dimensional gel analysis and two-dimensional tryptic phosphopeptide mapping support the identity of the basic, chromatin-associated protein and S6. Our experimental data indicate the existence of a nuclear pool of S6 whose phosphorylation is hormone inducible.