HOMOLOGOUS SEGMENTS IN 3 SUBUNITS OF THE GUANINE-NUCLEOTIDE EXCHANGE FACTOR EIF2B MEDIATE TRANSLATIONAL REGULATION BY PHOSPHORYLATION OF EIF2

eIF2B is a five-subunit guanine nucleotide exchange factor that is neg atively regulated by phosphorylation of the alpha subunit of its subst rate, eIF2, leading to inhibition of translation initiation, To analyz e this regulatory mechanism, we have characterized 29 novel mutations in the homologous eIF2B subunits encoded by GCD2, GCD7, and GCN3 that reduce or abolish inhibition of eIF2B activity by eIF2 phosphorylated on its ex subunit [eIF2(alpha P)]. Most, if not all, of the mutations decrease sensitivity to eIF2(alpha P) without excluding GCN3, the none ssential subunit, from eIF2B; thus, all three proteins are critical fo r regulation of eIF2B by eIF2(alpha P). The mutations are clustered at both ends of the homologous region of each subunit, within two segmen ts each of approximately 70 amino acids in length, Several mutations a lter residues at equivalent positions in two or all three subunits, Th ese results imply that structurally similar segments in GCD2, GCD7, an d GCN3 perform related functions in eIF2B regulation, We propose that these segments form a single domain in eIF2B that makes multiple conta cts with the ex subunit of eIF2, around the phosphorylation site, allo wing eIF2B to detect and respond to phosphoserine at residue 51, Most of the eIF2 is phosphorylated in certain mutants, suggesting that thes e substitutions allow eIF2B to accept phosphorylated eIF2 as a substra te for nucleotide exchange.