A HEAT-SENSITIVE ARABIDOPSIS-THALIANA KINASE SUBSTITUTES FOR HUMAN P70(S6K) FUNCTION IN-VIVO

In mammalian cells, mitogen-induced phosphorylation of ribosomal prote in S6 by p70(s6k) has been implicated in the selective translation upr egulation of 5'TOP mRNAs. We demonstrate here that the homologous Arab idopsis thaliana protein, AtS6k2, ectopically expressed in human 293 c ells or isolated from plant cells, phosphorylates specifically mammali an and plant S6 at 25 degrees C but not at 37 degrees C. When Arabidop sis suspension culture cells are shifted from 25 to 37 degrees C, the kinase becomes rapidly inactivated, consistent with the observation th at heat shock abrogates S6 phosphorylation in plants. Treatment with p otato acid phosphatase reduced the specific of immunoprecipitated AtS6 k2 threefold, an effect which was blocked in the presence of 4-nitroph enyl phosphate. In quiescent mannalian cells, AtS6k2 is activated by s erum stimulation, a response which is abolished by the fungal metaboli te wortmannin but is resistant to rapamycin. Treatment of mannalian ce lls with rapamycin abolished in vivo S6 phosphorylation by p70(s6k); h owever, ectopic expression of AtS6k2 rescues the rapamycin block. Coll ectively, the data demonstrate that AtS6k2 is the functional plant hom olog of mammalian p70(s6k) and identify a new signaling pathway in pla nts.