P70(S6K) FUNCTION IS ESSENTIAL FOR G1-PROGRESSION

AN essential step in the pathway by which growth factors trigger cellu lar proliferation is the induction of high levels of protein synthesis 1-3. This appears in part to be controlled by multiple phosphorylation of the ribosomal protein S6 (refs 4, 5). The main kinase responsible, p70s6k (refs 6-8), is activated through the phosphorylation of four s ites clustered in a putative autoinhibitory domain9, which is mediated by a signalling pathway distinct from those used by other well charac terized mitogen-activated serine/threonine kinases (such as p42/p44map k or p90rsk; refs 10, 11). Here we investigate the role of p70s6k in t he mitogenic response. Microinjection of quiescent rat embryo fibrobla sts with any of three distinct polyclonal antibodies to p70s6k abolish es serum-induced entry into S phase of the cell cycle. This effect is preceded by almost complete abrogation of the activation of protein sy nthesis and the expression of an essential immediate early gene produc t, c-fos. The inhibitory effect on DNA synthesis is also elicited by m icroinjection of the antibodies late in G1 phase, consistent with the finding that p70s6k activity remains high throughout G1.