Nucleocytoplasmic Transport of Ribosomal Protein S6

Abstract

Ribosomal protein S6 in the 40S subunit of higher eukaryotic cells is remarkable for its ability to exist in a variety of states of phosphorylation (Gressner, Wool, 1974; Thomas et al, 1979; Kruppa et al, 1983). Up to 5 mol of phosphate/mol of S6 protein are incorporated upon stimulation of cells by cycloheximide (Krieg et al, 1988), growth factors and oncogene products (Erikson, 1991; Sturgill, Wu, 1991). Results in vivo and in vitro indicate that S6 phosphorylation leads to activation of translation and cell growth by facilitating the initiation process of protein synthesis (Duncan, McConkey, 1982; Thomas et al, 1982). The phosphate group is transferred to specific serine residues at the C-terminal domain of S6 (Krieg et al, 1988) by different kinases, e.g. cAMP dependent protein kinase and S6 kinase. The mitogen-activated S6 kinase is able to phosphorylate four of the five sites in vivo (Krieg et al, 1988).