Stoichiometric association of cap-binding protein I with translated polysomal globin mRNP.

The protein composition of a 12S polysomal globin messenger ribonucleoprotein (pmRNP) from rabbit reticulocytes was examined. The pmRNP was released from purified polysomes by puromycin treatment under run-off conditions of protein synthesis. The protein pattern of this pmRNP depends on the potassium ion concentration used during the run-off and the subsequent isolation. Several proteins show a salt-dependent association with the pmRNP while a few are constituents of the pmRNP at all salt concentrations tested. By cross-linking the pmRNP-derived proteins to [3H]methyl-labelled oxidized vesicular stomatitis virus (VSV) mRNA and by immunoblotting against anti-cap-binding protein (CBP I) antibodies, it is demonstrated that the association of the CBP I with the pmRNP depends on the ionic strength. At 65 mM KCl, CBP I shows low affinity for the pmRNP; at 140 mM KCl, the affinity of CBP I for the pmRNP is greatly enhanced. At this ionic strength, equimolar amounts of CBP I and mRNA are found in the pmRNP. At 500 mM KCl, the pmRNP is completely devoid of CBP I. In the non-translated free cytoplasmic mRNP (cmRNP) no CBP can be detected by either the cross-link or the immunoblot technique.