Casein kinase II catalyzes tyrosine phosphorylation of the yeast nucleolar immunophilin Fpr3

In the yeast Saccharomyces cerevisiae, the nucleolar immunophilin, Fpr3, is phosphorylated at tyrosine and dephosphorylated by the phosphotyrosine-specific phosphoprotein phosphatase, Ptp1, In Ptp1-deficient cells, Fpr3 contains phospho-Tyr at a single site (Tyr(184), but also contains phospho-Ser and phospho-Thr, Ser(186) (adjacent to Tyr(184)) is situated within a canonical site for phosphorylation by casein kinase II (CKII), Yeast cell lysates contain an activity that binds to Fpr3 in vitro and phosphorylates Fpr3 at Ser, Thr, and Tyr; this activity was found to be dependent on expression of functional yeast CKII, Moreover, purified Fpr3 was phosphorylated on Tyr(184) in vitro by either purified yeast CKII or purified, bacterially-expressed human CKII, Likewise, phosphorylation of Fpr3 at tyrosine in vivo was markedly enhanced in yeast cells overexpressing a heterologous (Drosophila) CKII, but was undetectable in yeast cells carrying only a temperature-sensitive allele of the endogenous CKII, even when the cells were grown at a permissive temperature. Phosphorylation of Fpr3 at Tyr(184) by CKII in vitro lagged behind phosphorylation of Fpr3 at Ser, and was accelerated by pre-phosphorylation of Fpr3 at Ser using CKII, Furthermore, synthetic peptides corresponding to the sequence surrounding Tyr(184) that contained P-Ser (or Glu) at position 186 were much more efficient substrates for CICTI phosphorylation of Tyr(184) than a synthetic peptide containing Ala at position 186. These findings indicate that CKII phosphorylates Fpr3 at tyrosine and serine both in vivo and in vitro and thus possesses dual specificity, These results also indicate that Tyr(184) is phosphorylated by CKII via a tao-step process, in which phosphorylation at the +2 position provides a negatively-charged specificity determinant that allows subsequent phosphorylation of Tyr(184).