A key control point in the initiation of protein synthesis in mammalian cells is the recycling of eukaryotic initiation factor (eIF)-2 by the guanine nucleotide exchange factor eIF-2B. In mammalian cells, eIF-2B is a complex of five different subunits termed epsilon, delta, gamma, beta and alpha. To clone cDNAs for the beta subunit of rabbit eIF-2B, amino acid sequence data was first obtained and used to design redundant oligonucleotide primers for use in PCR. PCR products were used to screen a rabbit liver cDNA library in lambda gt11 to obtain full-length cDNAs for eIF-2B beta. The cDNAs were sequenced completely on both strands and revealed an open reading frame encoding a predicted 351-amino acid polypeptide of 39.0 kDa. The molecular mass and pI (5.99) of the predicted protein agree well with the properties of eIF-2B beta purified from rabbit reticulocytes. In vitro transcription/-translation of the cDNAs gave rise to a product that migrated at a position indistinguishable from that of this subunit of the purified protein. The amino acid sequence shows a high degree of similarity to that of GCD7, a Saccharomyces cerevisiae protein thought to be equivalent to mammalian eIF-2B beta. Northern-blot analysis revealed a single major mRNA species for eIF-2B beta in each of the four rabbit tissues tested.
Photoaffinity labeling of the rabbit reticulocyte guanine nucleotide exchange factor and eukaryotic initiation factor 2 with 8-azidopurine nucleotides