Multiple Mechanisms Control Phosphorylation of PHAS-I in Five (S/T)P Sites That Govern Translational Repression

Abstract

Control of the translational repressor, PHAS-I, was investigated by expressing proteins with Ser/Thr → Ala mutations in the five (S/T)P phosphorylation sites. Results of experiments with HEK293 cells reveal at least three levels of control. At one extreme is nonregulated phosphorylation, exemplified by constitutive phosphorylation of Ser82. At an intermediate level, amino acids and insulin stimulate the phosphorylation of Thr36, Thr45, and Thr69 via mTOR-dependent processes that function independently of other sites in PHAS-I. At the third level, the extent of phosphorylation of one site modulates the phosphorylation of another. This control is represented by Ser64 phosphorylation, which depends on the phosphorylation of all three TP sites. The five sites have different influences on the electrophoretic properties of PHAS-I and on the affinity of PHAS-I for eukaryotic initiation factor 4E (eIF4E). Phosphorylation of Thr45 or Ser64 results in the most dramatic decreases in eIF4E binding in vitro. However, each of the sites influences mRNA translation, either directly by modulating the binding affinity of PHAS-I and eIF4E or indirectly by affecting the phosphorylation of other sites.

ACKNOWLEDGMENTS

This research was supported in part by National Institutes of Health grants DK52753 and DK28312 (to J.C.L.), NIH grants HL19242 and DK52378 (to T.A.J.H.), and a fellowship from the Juvenile Diabetes Foundation (to I.M.-S.).

We thank John Majors for p2332, Kevin Bowman for expert technical assistance, and Gregory Brunn and Angus Scrimgeour for their critical reading of the manuscript.