Preparation of a cell-free translation system with minimal loss of initiation factor eIF-2/eIF-2B activity

Cell-free translation systems prepared from suspension-cultured HeLa S3 cells or mouse L cells by hypotonic shock followed by Dounce homogenization poorly initiated the translation of exogenous mRNA. In contrast, cell extracts prepared from cells exposed to the detergent lysolecithin translated exogenous mRNA readily. The block in initiation in the former lysates was localized to the ribosome fraction. During in vitro translation polysomes from homogenized cells disaggregated but the run-off ribosomes were unable to reinitiate translation. The block resulted from a decrease in eukaryotic initiation factor 2 (eIF-2) or the guanine nucleotide exchange factor (eIF-2B) activity, since the addition of eIF-2 or eIF-2B to these latter extracts substantially improved the capacity of the extract to initiate translation of exogenous mRNA. Extracts from homogenized cells, but not from detergent-treated cells, showed enhanced ability to phosphorylate the alpha subunit of exogenous eIF-2. We show that the method of cell extract preparation greatly influences the state of eIF-2/eIF-2B activity in the resulting extract and that extracts in which this activity is maintained can readily initiate translation on exogenous mRNA and reinitiate on endogenous mRNA.