Purification and properties of a protein component of messenger ribonucleoprotein particles that shares a common epitope with eucaryotic elongation factor Tu

A 62‐kDa polypeptide, which reacts with antibodies directed against a peptide corresponding to a portion of the amino‐terminal structure of eucaryotic elongation factor Tu(eEF‐Tu), was purified from the 0.5 M NaCl wash of rabbit reticulocyte polysomes. Previous work has shown that this polypeptide is a constituent of messenger ribonucleoprotein particles (mRNPs) from a variety of mammalian cell types [Greenberg, J. R. and Carroll, E. C. (1985) Mol. Cell Biol. 5, 342–351]. The purified polypeptide bound mRNA as well as rRNA using a nitrocellulose‐filter assay. The same nitrocellulose‐filter assay failed to detect binding to GTP. Using a competition‐binding assay, it was established that the purified polypeptide interacts with poly(U) and poly(G) but not with poly(A). This preference for synthetic polynucleotides was the same as found for eEF‐Tu [Slobin, L. I. (1983) J. Biol. Chem. 258, 4895–4900]. Furthermore, treatment of the purified RNA‐binding protein with trypsin resulted in a rapid cleavage of two peptide bonds resulting in fragments of 60 kDa and 53 kDa. Trypsin also cleaves eEF‐Tu rapidly at two bonds resulting in two large polypeptide fragments [Slobin, L. I., Clark, R. V. & Olson, M. O. J. (1981) Biochemistry 20, 5761–5767]. The amino acid sequence of the first 39 residues of the purified RNA‐binding protein was determined and found to possess no homology to eEF‐Tu.