PURIFICATION AND PARTIAL CHARACTERIZATION OF AN ACIDIC RIBOSOMAL-PROTEIN KINASE FROM MAIZE

Phosphorylation and dephosphorylation of ribosomal proteins have been suggested to participate in the regulation of protein synthesis in euk aryotic organisms. The present research focuses on the purification an d partial characterization of a protein kinase from maize ribosomes th at specifically phosphorylates acidic ribosomal proteins. Ribosomes pu rified from maize axes were used as the enzyme source. Purification of ribosomes was performed by centrifugation through a 0.5 M sucrose, 0. 8 M KCl cushion. A protein kinase activity present in this fraction wa s released by extraction with 1.5 M KCl and further purified by diethy laminoethyl cellulose column chromatography. A peak containing protein kinase activity was eluted around 400 mM KCl. Analysis of this fracti on by sodium dodecyl sulfate polyacrylamide gel electrophoresis showed one band of 38 kDa molecular mass, which cross-reacted in a western b lot with antibodies raised against proteins from the large ribosomal s ubunit. This enzyme specifically phosphorylates one of the acidic ribo somal proteins (P-2). Its activity is inhibited by Ca2+ and Zn2+ and i s activated by Mg2+, polylysine and spermine. The relevance of this pr otein kinase in reinitiating the protein synthesis process during germ ination is discussed.