RIBOSOMAL ASSOCIATION OF POLY(A)-BINDING PROTEIN IN POLY(A)-DEFICIENTSACCHAROMYCES-CEREVISIAE

Poly(A) binding protein, the most abundant eukaryotic mRNP protein, is known primarily for its association with polyadenylate tails of mRNA. In the yeast, Saccharomyces cerevisiae, this protein (Pabp) was found to be essential for viability and has been implicated in models featu ring roles in mRNA stability and as an enhancer of translation initiat ion. Although the mechanism of action is unknown, it is thought to req uire an activity to bind poly(A) tails and an additional capacity for an interaction with 60 S ribosomal subunits, perhaps via ribosomal pro tein L46 (RpI46), We have found that a significant amount of Pabp in w ild-type cells is not associated with polyribosome complexes. The rema ining majority, which is found in these complexes, maintains its assoc iation even in yeast cells deficient in polyadenylated mRNA and/or RpI 46, These observations suggest that Pabp may not require interaction w ith poly(A) tails during translation, Further treatment of polyribosom e lysates with agents known to differentially disrupt components of po lyribosomes indicated that Pabp may require contact with some RNA comp onent of the polyribosome, which could be either non-poly(A)-rich sequ ences of the translated mRNA or possibly a component of the ribosome. These findings suggest that Pabp may possess the ability to bind to ri bosomes independently of its interaction with poly(A), We discuss thes e conclusions with respect to current models suggesting a multifunctio nal binding capacity of Pabp.