THE PHOSPHORYLATED RIBOSOMAL-PROTEIN S7 IN TETRAHYMENA IS HOMOLOGOUS WITH MAMMALIAN S4 AND THE PHOSPHORYLATED RESIDUES ARE LOCATED IN THE C-TERMINAL REGION - STRUCTURAL CHARACTERIZATION OF PROTEINS SEPARATED BY 2-DIMENSIONAL POLYACRYLAMIDE-GEL ELECTROPHORESIS

A single basic ribosomal protein, protein S7, can be multiply phosphor ylated in the ciliated protozoan Tetrahymena. Induction of phosphoryla tion is highly regulated, and the phosphorylation proceeds in a strict ly sequential manner. The first site to be phosphorylated is a serine residue and the second a threonine. In this paper we report the comple te primary structure of Tetrahymena thermophila ribosomal protein S7 i ncluding identification of the phosphorylated serine and threonine res idues. Most of the sequence information was obtained from peptides gen erated by in situ digestion of S7 in two-dimensional gels using an app roach that combined traditional protein chemistry with mass spectromet ry. T. thermophila ribosomal protein S7 has a molecular mass of 29,459 Da and contains 259 amino acid residues. Phosphorylation takes place on Ser(258) and Thr(248) in the C-terminal region of the protein. Alig nment of T. thermophila ribosomal protein S7 with known ribosomal prot eins yielded the surprising result that T. thermophila 57 is homologou s, not with mammalian ribosomal protein S6, but with mammalian ribosom al protein S4. These findings clearly distinguish the pattern of phosp horylation of ribosomal proteins in Tetrahymena from all other eukaryo tes analyzed to date.