Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation.
Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2-4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322-334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519-521]. In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2-4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2-4-fold increase in labelling of the eukaryotic initiation factor eIF-4 alpha with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4 alpha. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2-3-fold increase in eIF-4 alpha phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4 alpha kinase activity rather than phosphate turnover on eIF-4 alpha. Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4 alpha recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4 alpha and eIF-4 gamma into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.
Morley
S J
SJ
Biochemistry Laboratory, School of Biological Sciences, University of Sussex, Brighton, England.
Rau
M
M
Kay
J E
JE
Pain
V M
VM
D000595:Amino Acid Sequence; D000818:Animals; D001798:Blood Proteins / Q000096:biosynthesis; D003208:Concanavalin A / Q000494:pharmacology; D039601:Eukaryotic Initiation Factor-4A; D039562:Eukaryotic Initiation Factor-4F; D006801:Humans; D008213:Lymphocyte Activation; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010448:Peptide Initiation Factors / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D010766:Phosphorylation; D013552:Swine; D013601:T-Lymphocytes / Q000187:drug effects / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D001798:Blood Proteins; D039562:Eukaryotic Initiation Factor-4F; D010448:Peptide Initiation Factors; D010710:Phosphates; D003208:Concanavalin A; D039601:Eukaryotic Initiation Factor-4A; D013755:Tetradecanoylphorbol Acetate