Natural mRNA is required for directing Met-tRNA(f) binding to 40S ribosomal subunits in animal cells: involvement of Co- XXXX in natural mRNA-directed initiation complex formation.

@entity15204 @entity12465 @entity31952 @entity19601 @entity21178
Two protein factors, @entity15204 as well as a high molecular weight protein complex from reticulocyte ribosomal high-salt wash which we term Co- @entity15204 , promote Met-tRNA(f) binding to 40S ribosomes. This binding is dependent on the presence of an AUG codon or natural mRNAs [Roy et al. (1984) Biochem. Biophys. Res. Commun. 122, 1418-1425]. Co- @entity15204 contains two component activities, Co- @entity12465 and Co- @entity21178 . Previously, we have purified an 80-kDa polypeptide containing Co- @entity12465 activity and showed that this polypeptide is a component of Co- @entity15204 and is responsible for Co- @entity12465 activity in Co- @entity15204 [Chakravarty et al. (1985) J. Biol. Chem. 260, 6945-6949]. We now report purification of a protein complex (subunits of Mr 180K, 110K, 65K, 63K, 53K, 50K, 43K, and 40K) containing Co- @entity21178 activity and devoid of Co- @entity12465 activity. In SDS-PAGE, the purified Co- @entity21178 preparation and an @entity19601 preparation (purified in Dr. A. Wahba's laboratory) separated into seven similar major polypeptides (Mr 110K, 65K, 63K, 53K, 50K, 43K, and 40K). The 50-kDa polypeptide in Co- @entity21178 was immunoreactive with a monoclonal antibody against @entity31952 (50 kDa). We have studied the roles of purified Co- @entity12465 and Co- @entity21178 activities in ternary and Met-tRNA(f).40S ribosome complex formation. The results are as follows: (1) At low and presumably physiological factor concentration (30 nM), @entity15204 did not form detectable levels of ternary complex. Moreover, such complex formation was totally dependent on the presence of Co- @entity12465 and/or Co- @entity21178 .(ABSTRACT TRUNCATED AT 250 WORDS)
