The polyribosomal poly(A)‐binding protein is highly conserved in vertebrate species

In eukaryotic cells mRNA is present in the form of messenger ribonucleoprotein complexes (mRNP particles). When polyribosomes, from a variety of cell types and tissues, are dissociated with EDTA or puromycin, the released mRNA is found tightly associated with 3 major proteins ofM r ~ 43 000-52 000 and 72 000-78 000 (review The larger of these proteins was shown to interact with the 3'-polyadenylate sequence of mRNA [2,3] since it may be recovered with poly(A) following digestion of polyribosomes or purified mRNP with ribonucleases A and T1. From density measurements of the poly(A) RNP particle it may be estimated that ~4 5 copies of this protein are associated with the average-sized poly(A) fragment The affinity of the poly(A)-binding protein for poly(A) is very high since it resists not only 0.5 M KC1 [1] but also centrifugation through Cs2SO4 density gradients Furthermore, this protein binds not only poly(A) but also regions of poly(A) adjacent and non-adjacent to poly(A) [81.