Eukaryotic initiation factor-3 (eIF3) is the largest of the eIFs and consists of at least 10 nonidentical subunits in mammals. See p66 (EIF3S7). The S. cerevisiae Prt1, or p90, protein is an integral subunit of eIF3. Mutations in Prt1 impair translation initiation at 37 degrees Celsius. The predicted 873-amino acid human PRT1 protein is 31% identical to yeast Prt1. PRT1 is an acidic protein, with a predicted pI of 4.8, and contains an RNA recognition motif (RRM). Immunologic experiments demonstrated that PRT1 corresponds to the 115-kD polypeptide of eIF3 and is part of the eIF3 complex. PRT1 interacts directly with the p170 subunit of eIF3, and the region required for this association is located within the RRM. Northern blot analysis revealed that PRT1 is expressed as a 3.1-kb mRNA in HeLa cells.