Differential subcellular localisation of two isoforms of p70 S6 protein kinase.

Ribosomal protein S6 phosphorylation is stimulated by a variety of hormones and growth factors. In mammalian cells the predominant S6 kinase is a 70 kDa enzyme which exists in two forms differing only in their amino terminal region. To determine whether this difference is functionally significant we have examined the intracellular localisation of the two forms. In cells transfected with cDNA encoding the longer alpha I p70 S6 kinase, protein is found in both the cytoplasmic and nuclear compartments. In contrast, the shorter alpha II polypeptide is confined to the cytoplasm. The significance of this differential localisation is discussed with respect to the recent identification of p70 S6 kinase activity as being essential for cell-cycle progression and thus mitogenesis.