Ser-64 and Ser-111 in PHAS-I Are Dispensable for Insulin-stimulated Dissociation from eIF4E

Insulin stimulates phosphorylation of multiple sites in the eIF4E-binding protein, PHAS-I, leading to dissociation of the PHAS-I·eIF4E complex and to an increase in cap-dependent translation. The Ser-64 and Ser-111 sites have been proposed to have key roles in controlling the association of PHAS-I and eIF4E. To determine whether the effects of insulin require these sites, we assessed the control of PHAS-I proteins having Ala-64 or Ala-111 mutations. The results indicate that phosphorylation of neither site is required for insulin to promote release of PHAS-I from eIF4E. Also, the mutation of Ser-111, which has been proposed to serve as a necessary priming site for the phosphorylation of other sites in PHAS-I, did not affect the phosphorylation of Thr-36/45, Ser-64, or Thr-69. Insulin promoted the release of eIF4E from PHAS-II, a PHAS isoform that lacks the Ser-111 site, but it was without effect on the amount of eIF4E bound to the third isoform, PHAS-III. The results demonstrate that contrary to widely accepted models, Ser-64 and Ser-111 are not required for the control of PHAS-I binding to eIF4E in cells, implicating phosphorylation of the Thr sites in dissociation of the PHAS-I·eIF4E complex. The findings also indicate that PHAS-II, but not PHAS-III, contributes to the control of protein synthesis by insulin.