Initiation of protein synthesis during lymphocyte stimulation

Abstract

THE stimulation of protein synthesis that accompanies the activation of peripheral blood lymphocytes by phytohaemagglutinin (PHA) has been shown to be due largely to an increase in the rate of initiation of protein synthesis¹. Such an increase could result from either an increase in the availability of initiation factors or of mRNA. This problem can be approached most directly by comparing the ability of cell-free protein synthesising systems from stimulated and unstimulated lymphocytes to form initiation complexes with ³⁵S-methionyl-tRNA_f^Met. Such studies in reticulocytes have shown that the limitation of initiation occurring during haemin deprivation is accompanied by a marked reduction in the number of methionyl-tRNA-40S ribosomal subunit complexes present, and enabled the elucidation of the mechanisms involved^2,3. We show here that the binding of ³⁵S-methionyl-tRNA_f^Met to both 40S subunits and 80S ribosomes is increased after activation of lymphocytes by PHA. These differences can be overcome by the addition of initiation factors from reticulocyte ribosomes, but not by the addition of exogenous globin mRNA.