A 75,000-dalton protein has been purified approximately 1,000-fold from rat liver, based on its capacity to organize poly(A) in a 27-residue repeating structure. This protein may be identified with the major polypeptide component of cytoplasmic poly(A)-ribonucleoprotein (RNP) previously described. The poly(A)-organizing activity of the protein is detected only in cytoplasmic fractions. Upon nuclease digestion of the 75,000-dalton protein-poly(A) complex, monomers, and higher multimers of RNP subunits can be resolved in a sucrose gradient. The sedimentation rate of the monomer is compatible with a composition of one 75,000-dalton protein molecule and one 27-residue segment of poly(A).

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