Sequence of a phosphorylation site in nucleolar protein B23

The sites of phosphorylation in protein B23, a silver-staining preribosomal ribonucleoprotein particle protein, were analyzed by tryptic peptide mapping. Three 32P peptides were found using in vitro labeling of nucleoli. An additional unlabeled phosphopeptide was identified by amino acid analysis. The sequence of the latter was Asp-Thr(P)-Pro-Ala-Lys. These results suggest that protein B23 contains one class of site labeled rapidly in vitro and another type of site phosphorylated only in vivo.