Translational initiation factor eIF-2 subcellular levels and phosphorylation status in the developing rat brain.

We have quantified the levels of the alpha subunit of initiation factor 2 (eIF-2) in the postmicrosomal supernatant and the ribosomal salt wash fractions from suckling and adult rat brain. The levels of eIF-2 in the ribosomal salt wash decrease in adult with respect to that present in suckling rat brain, but the total amount remains fairly constant, and a very close parallelism exists between the eIF-2 associated with ribosomes and RNA levels in the microsomal fraction in the two age groups. The phosphorylation state of eIF-2 alpha, as determined by isoelectric focusing followed by protein immunoblotting, in the same subcellular fractions, did not reveal the presence of the phosphorylated form in any of the fractions studied. These results suggest that phosphorylation of the alpha subunit is not implied in the regulation of protein synthesis initiation during brain development, and some other component regulates both the number of active ribosomes and eIF-2 levels in microsomes.