The ribosome biogenesis factor Arx1p: characterization of its recycling mechanism and its role in ribosome export

Translation is an essential and fundamental process that coverts genetic codes into functional polypeptides by an apparatus called ribosome. In eukaryotic cells, ribosomes are composed of two subunits: the large (60S) subunit and small (40S) subunits. In Saccharomyces cerevisiae, ribosome biogenesis is complex and requires the involvement of over ~170 trans-acting factors. As a growing number of factors were identified related to this essential metabolic pathway, our lab has contributed to functional characterization of the late 60S subunit biogenesis pathway that centers on Nmd3p. This work particularly focuses on characterizing of the nuclear shuttling trans-acting factor Arx1p found in the Nmd3p-60S subunit particle. A working model that describes how Rei1p, another cytosolic trans-acting factor, recycles Arx1p is presented. This work also shows a similar mode of Arx1p recycling by the Hsp40 J-protein, Jjj1p. Furthermore, I have investigated functional interplay between Arx1p and Rpl25p, a 60S ribosomal protein at the polypeptide exit tunnel. These findings further reveal the involvement of Arx1p at the polypeptide exit tunnel in mediating association of other factors with 60S subunits. Beyond its function at the polypeptide exit tunnel, this work also focuses on a function for Arx1p in the export of 60S subunits. In yeast and higher eukaryotes, 60S subunit export depends on the export adaptor Nmd3p via Crm1-dependent pathway. I show that ARX1 interacts with the NES of Nmd3p and nucleoporins. From these results, I propose that Arx1p acts as another export receptor to facilitate 60S subunit export.