EIF2B, THE GUANINE NUCLEOTIDE-EXCHANGE FACTOR FOR EUKARYOTIC INITIATION-FACTOR-2 - SEQUENCE CONSERVATION BETWEEN THE ALPHA-SUBUNIT, BETA-SUBUNIT AND DELTA-SUBUNIT OF EIF2B FROM MAMMALS AND YEAST

The guanine nucleotide-exchange factor eIF2B mediates the exchange of GDP bound to translation initiation factor eIF2 for GTP. This exchange process is a key regulatory step for the control of translation initi ation in eukaryotic organisms. To improve our understanding of the str ucture, function and regulation of eIF2B, we have obtained and sequenc ed cDNA species encoding all of its five subunits. Here we report the sequences of eIF2B beta and delta from rat. This paper focuses on sequ ence similarities between the alpha, beta and delta subunits of mammal ian eIF2B. Earlier work showed that the amino acid sequences of the co rresponding subunits of eIF2B in the yeast Saccharomyces cerevisiae (G CN3, GCD7 and GCD2) exhibit considerable similarity. We demonstrate th at this is also true for the mammalian subunits. Moreover, alignment o f the eIF2B alpha, beta and delta sequences from mammals and yeast, al ong with the sequence of the putative eIF2B alpha subunit from Caenorh abditis elegans and eIF2B delta from Schizosaccharomyces pombe shows t hat a large number of residues are identical or conserved between the C-terminal regions of all these sequences. This strong sequence conser vation points to the likely functional importance of these residues. T he implications of this are discussed in the light of results concerni ng the functions of the subunits of eIF2B in yeast and mammals. Our re sults also indicate that the large apparent differences in mobility on SDS/PAGE between eIF2B beta and delta subunits from rat and rabbit ar e not due to differences in their lengths but reflect differences in a mino acid composition. We have also examined the relative expression o f mRNA species encoding the alpha, beta, delta and epsilon subunits of eIF2B in a range of rat tissues by Northern blot analysis. As might b e expected for mRNA species encoding subunits of a heterotrimeric prot ein, the ratios of expression levels of these subunits to one another did not vary between the different rat tissues examined (with the poss ible exception of liver). This represents the first analysis of the le vels of expression of mRNA species encoding the different subunits of eIF2B.