Activation of p70/p85 S6 kinase by a pathway independent of p21fi ras

THE enzymes p70^s6k and p85^s6k are two isoforms of the same kinase^1,2 and are important in mitogenesis^2-4. Both isoforms are activated by a complex phosphorylation event5 and lie on a common signalling pathway⁴, distinct from that of the p42^mapk/ p44^mapk kinases⁶ Activation of P42^mapkp44 ^mapk is triggered by sequential activation of the GDP- GTP exchange factor Sos, the GTP-binding protein p21^ras, and protein kinases p74^rafand p47^mek (refs 7-10). As p21^ras transformed cells have increased S6 phosphorylation¹¹, we tested whether the p70^s6k/p85^s6k signalling pathway bifurcates between p21^ras and P42^mapk/p44^mapk. We found that mutants of p74^raf and p2^ras blocked activation of epitope-tagged p44^mapk but not epitope-tagged p70^s6k. Moreover, in cells expressing human platelet-derived growth factor receptors lacking the kinase-insert domain, the growth factor activates p21^ras but not p70^s6k/p85^s6k. The critical autophosphorylation site for p70^s6k/ p85^s6k activation within this domain is a tyrosine at residue 751. Our results show that the p70^s6k/p85 ^s6k signalling pathway is independent of p21^ras, that it bifurcates from the p2l^ras pathway at the receptor, and that it is initiated by autophosphorylation at a specific site.