Cross‐linked proteins associated with a specific mRNA in the cytoplasm of HeLa cells

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Abstract
Cytoplasmic mRNA of eukaryotic cells is distributed between polysomal and post-polysomal fractions (free) as protein-bound complexes [RNP]. These studies were designed to determine whether a specific mRNA isolated from different subcellular compartments is complexed with the same family of polypeptides. As a first approach, the proteins associated with mRNA which codes for histone H4 were examined. To perform these experiments [human cervical carcinoma] HeLa cells were exposed to UV light to cross-link in vivo polypeptides which are closely associated with nucleic acid. To identify the polypeptides associated with mRNA specific for histones a genomic probe for histone H4 mRNA was immobilized on epoxy-cellulose. By hybrid selection specific mRNP containing histone mRNA were isolated. There are a number of polypeptides associated with both polysomal and post-polysomal histone mRNA. In polysomal histone mRNA 2 polypeptides of MW = 49,000 and 52,500 were the major components. In contrast polypeptides of MW = 43,000 and 57,000 were the major polypeptide components of post-polysomal (or free) histone mRNA. Furthermore, the polypeptides associated with either polysomal or free H4 histone mRNA represent a subset of proteins found in poly(A)-free fractions or poly(A)-rich mRNA fractions.