Poly(U) and transfer RNA binding sites on reticulocyte ribosomes

Abstract

The alkylating agent 2-iodoacetamide was used to identify several protein components of mammalian ribosomes which were either masked or exposed by components of the aminoacyl transfer RNA binding reaction. When ribosomes and polyuridylic acid were allowed to form a complex, one iodoacetamide-reactive site of the smaller (40-S) ribosomal subunit was found to become more accessible to that reagent. Subsequent addition of aminoacyl and/or deacylated transfer RNA largely reversed this effect. Another protein fraction, this one a component of the larger (60-S) subunit, was significantly masked by either transfer RNA preparation; the effect was polyuridylic acid-dependent. In the intact uncomplexed ribosome, the site associated with polyuridylic acid appears to be relatively inaccessible to alkylation. This implies either that it is located in the interior of the ribosome or else it is more superficial but masked by other components of the ribosome.