Eukaryotic translation initiation factor 3 (elF3) is a large multisubunit protein complex that plays an essential role in the binding of the initiator methionyl-tRNA and mRNA to the 40S ribosomal subunit to form the 40S initiation complex. cDNAs encoding all the subunits of mammalian elF3 except the p42 subunit have been cloned in several laboratories. Here we report the cloning and characterization of a human cDNA encoding the p42 subunit of mammalian elF3. The open reading frame of the cDNA, which encodes a protein of 320 amino acids (calculated M(r) 35 614) has been expressed in Escherichia coli and the recombinant protein has been purified to homogeneity. The purified protein binds RNA in agreement with the presence of a putative RNA binding motif in the deduced amino acid sequence. The protein shows 33% identity and 53% similarity with the Tif35p subunit (YDR 429C) of yeast elF3. Transfection experiments demonstrated that polyhistidine-tagged p42 protein, transiently expressed in human U20S cells, was incorporated into endogenous elF3. Furthermore, elF3 isolated from transfected cell lysates contains bound elF5 indicating that a specific physical interaction between elF5 and elF3 may play an important role in the function of elF5 during translation initiation in eukaryotic cells.
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