Loss of hydrophobic motif and activation loop phosphorylation is a consequence and not the mechanism of S6 kinase 1 inhibition by rapamycin.

S6K1 regulation associates a central role with dynamics of sequential phosphorylations at the hydrophobic motif (T412) and activation loop (T252) of the enzyme, such that the hydrophobic motif phosphorylation supposedly brought about by mTOR- kinase, primes the enzyme for PDK1 dependent phosphorylation at the activation loop for its full activation. Accordingly loss of hydrophobic motif phosphorylation attributed to TOR- kinase inhibition, with resultant loss of activation loop phosphorylation is the hypothesis put forward to explain the mechanism of rapamycin inhibition. Our recent observation that rapamycin continues to inhibit S6K1 in the absence of either phosphorylation, together with the evidence that phosphorylation at activation loop may occur prior to that of hydrophobic motif raises serious questions about the proposed mechanism of rapamycin inhibition. Here, we show that rapamycin fails to effect preferential loss of either phosphorylation and the two instead exhibit equal sensitivity to rapamycin both in time and quantum. We further show that of activation loop and hydrophobic motif phosphorylations turnover in an interdependent manner so as to exhibit all or none pattern of loss to rapamycin. Using insect cell expression system, we further substantiate their interdependent turnover and provide evidence that the two phosphorylations are brought about in a coordinate and not sequential manner. These data together with the observation that both kinases that cause hydrophobic motif and activation loop phosphorylations in insect or mammalian cells are completely insensitive to inhibition by rapamycin, suggest that their loss is a consequence and not the mechanism of rapamycin inhibition in accordance with the model proposed herein.