The polyribosomal poly(A)-binding protein is highly conserved in vertebrate species: Comparison in duck, mouse and rabbit

Abstract

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 o fM r ~ 43 000-52 000 and 72 000-78 000 (review [1]). 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 [4]. 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 [5,6] and affords protection ofpoly(A) against ribonucleases and nucleases [7 10]. Furthermore, this protein binds not only poly(A) but also regions of poly(A) adjacent and non-adjacent to poly(A) [81. A protein of the same M r as the polyribosomal poly(A)-binding protein has been observed to be associated with the 3'-poly(A)region of nuclear pre-