Amino acid sequence and sites of phosphorylation in a highly acidic region of nucleolar nonhistone protein C23

Novikoff hepatoma nucleolar nonhistone proteins, C23 and B23, contain highly acidic phosphorylated regions (Mamrack, M. D., et al. (1977) Biochem. Biophys. Res. Commun. 76, 150--157). Tryptic peptides from protein C23 containing these regions were purified by DEAE-Sephadex columns and paper electrophoresis at pH 1.8. One of these, peptide C23-Ca, was sequenced by combined automated and conventional methods. The proposed amino acid sequence is shown in eq 1. This peptide was found in three 32P-labeled forms with phosphoryl groups at positions 8 and 25, and probably 28. The highly acidic sequences adjacent to the phosphorylation sites represent a unique class of phosphorylation sites different from those in histones or substrates for cytoplasmic cAMP-dependent kinases. Ala-Ala-Pro-Ala-A5la-Pro-Ala-Ser-Glu-A10sp-Glu-Asp-Glu-Glu-A15sp-Asp-Asp-Asp-Glu-A20sp-Asp-Asp-Asp-Asp-S25er-Gln-Glu-Ser-Glu-G30lu-Glu-Asp-Glu-Glu-V35al-Met-Glu-Ile-Thr-P40ro-Ala-Lys (1).