CONSTITUTIVE ACTIVATION OF S6 KINASE BY DELETION OF AMINO-TERMINAL AUTOINHIBITORY AND RAPAMYCIN SENSITIVITY DOMAINS

The mitogen response of p70/p85 S6 kinase (S6K) parallels that of mito gen-activated protein kinases (MAPK). However, S6K lies on a discrete signaling pathway from MAPK, since the immunosuppressant drug rapamyci n inactivates S6K without affecting the MAPK cascade. Phosphatidylinos itol 3-kinase operates upstream of S6K, but the intermediate effecters in this signaling pathway are unknown. We have identified an autoinhi bitory domain in S6K that overrides the requirement of the amino termi nus for the activation of S6K. The region between codons 58 and 77 is highly inhibitory, and its deletion results in constitutive kinase act ivation. Additionally, deletion of the first 77 codons confers mitogen independence and insensitivity to rapamycin. Rat1 cells expressing De lta N77 S6K exhibit a distinctly abnormal morphology. This constitutiv ely active mutant will provide a useful means of studying the effects of expressing unregulated S6K in cells. Subdeletion analysis of the am ino terminus has defined two discrete domains in the N terminus of S6K -a domain between codons 1 and 58 is essential for the mitogen activat ion of S6K and confers rapamycin sensitivity; a second domain between codons 58 and 77 confers autoinhibition. We propose a model for the ac tivation of S6 kinase in which mitogen-stimulated cellular factors int eract with the amino terminus to negate the effects of the autoinhibit ory domain.