RECONSTITUTION AND PURIFICATION OF EUKARYOTIC INITIATION-FACTOR 2B (EIF2B) EXPRESSED IN SF21 INSECT CELLS

Eukaryotic initiation factor eIF2B plays a key role in the regulation of protein synthesis through its ability to catalyze the exchange of G DP bound to a second initiation factor, eIF2, for free GTP. In contras t to other GDP-GTP exchange factors (GEFs), which are often single sub unit proteins, eIF2B consists of five dissimilar subunits. In the stud ies reported here the baculovirus expression vector system (BEVS) was used to express FLAG epitope tagged alleles for the alpha, beta, gamma , delta, and epsilon subunits of rat eIF2B in Sf21 cells. The eIF2B ho loprotein was reconstituted in vivo by coexpression of all five subuni ts in Sf21 cells and was subsequently purified to greater than 98% hom ogeneity using a two-step procedure involving an anti-FLAG immunoaffin ity column followed by gel filtration chromatography, The purified fiv e-subunit eIF2B complex had high GEF activity as assayed by using [H-3 ]GDP-bound to eIF2 as a substrate. Alternatively eIF2B with high GEF a ctivity was reconstituted in vitro by mixing crude cell lysates contai ning different eIF2B subunits. The latter results suggest that eIF2B a ctivity in vivo could involve alterations in the concentration and/or the availability of individual subunits for holoprotein assembly. Over all, the results show the utility of the baculovirus-insect cell syste m for the expression, assembly, and purification of active recombinant multisubunit factors. (C) 1998 Academic Press.