Polysome-dependent in vitro translation system capable of peptide chain reinitiation

Detection of early gene expression changes during activation of human primary lymphocytes by in vitro synthesis of proteins from polysome-associated mRNAs

The rapid increase in protein synthesis during the mitogenic stimulation of human peripheral blood lymphocyte is the result of global and specific translational control mechanisms. To study some of these mechanisms, we examined the in vitro translatability of mRNAs associated with the polyribosome fraction. Polyribosome fractions were isolated from lymphocytes after activation with ionomycin and the phorbol ester PMA. The associated PAmRNAs were translated in the presence of mRNA-depleted rabbit reticulocyte lysate and [(35)S]Met, and the protein products were analyzed by SDS--PAGE and autoradiography. There was little synthesis of protein from the PAmRNAs isolated from unactivated T cells, but the PAmRNAs isolated from activated T cells showed a rapid increase in translatability. Translation of the PAmRNAs was sensitive to edeine and m7GTP, suggesting their cap-dependent translation. With activation, the majority of proteins showed increasing in vitro translation, but two proteins, p72 and p33, were found to have increased synthesis within 30 min, which decreased in 1 h. Transcription inhibitors were used to ascertain if regulation of their expression was transcriptional or translational. To identify these proteins, we used biotinylated lysine during the in vitro translation reaction, and we extracted the biotinylated protein by using streptavidin magnetic beads. The protein product was analyzed by mass spectrometry. p33 was identified as a prohibitin-like protein (BAP37), but the identification of p72 was not found in the databases. The distinct up-regulation and down-regulation of their protein expression suggest their tightly controlled regulation during early T cell activation.

Optimisation of protein synthesis rates in reticulocyte lysates and reconstituted systems

Means of increasing the very low activity of a reconstituted protein synthesising system from rabbit reticulocytes were investigated. Increasing the concentration of labelled amino acid, addition of polyamines, use of Sepharose-filtered as opposed to centrifuged ribosomes, use of untreated as opposed to gel-filtered cytosol and an increase in ratio of cytosol to ribosomes all contributed to the increase in activity of the system to the point where activity was clearly consistent with initiation taking place. Similar activities could not be attained with rat liver cytosol though rat liver ribosomes incorporated well in reticulocyte cytosol. Incorporation by lysates was also found to be dependent on the concentration of the labelled amino acid added.