Poly(A) tail-dependent exonuclease AtRrp41p from Arabidopsis thaliana rescues 5.8 S rRNA processing and mRNA decay defects of the yeast ski6 mutant and is found in an exosome-sized complex in plant and yeast cells

Eukaryotic 3'-->5' exonucleolytic activities are essential for a wide varie ty of reactions of RNA maturation and metabolism, including processing of r RNA, small nuclear RNA, and small nucleolar RNA, and mRNA decay. Two relate d but distinct forms of a complex containing 10 3'-->5' exonucleases, the e xosome, are found in yeast nucleus and cytoplasm, respectively, and related complexes exist in human cells. Here we report on the characterization of the AtRrp41p, an Arabidopsis thaliana homolog of the Saccharomyces cerevisi ae exosome subunit Rrp41p (Ski6p). Purified recombinant AtRrp41p displays a processive phosphorolytic exonuclease activity and requires a single-stran ded poly(A) tail on a substrate RNA as a "loading pad." The expression of t he Arabidopsis RRP41 cDNA in yeast rescues the 5.8 S rRNA processing and 3' -->5' mRNA degradation defects of the yeast ski6-100 mutant. However, neith er of these defects can explain the conditional lethal phenotype of the ski 6-100 strain. Importantly, AtRrp41p shares additional function(s) with the yeast Rrp41p which are essential for cell viability because it also rescues the rrp41 (ski6) null mutant. AtRrp41p is found predominantly in a high mo lecular mass complex in Arabidopsis and in yeast cells, and it interacts in vitro with the yeast Rrp44p and Rrp4p exosome subunits, suggesting that it can participate in evolutionarily conserved interactions that could be ess ential for the integrity of the exosome complex.