Biochemical Studies Of The Eukaryotic Rna Exosome

The RNA exosome is a multi-subunit protein complex involved in RNA maturation, surveillance, and turnover of cellular RNA. It is composed of a nine subunit core that contains an RNA binding central channel and associates with the additional subunit Rrp44. Rrp44 contains a 3' to 5' processive exoribonuclease activity and an endoribonuclease activity in its N-terminal PIN domain. The exosome is present in both the nucleus and cytoplasm. In the nucleus, the exosome associates with a second nuclease, Rrp6, which is a distributive 3' to 5' exoribonuclease. Although a wealth of studies have helped to characterize the exosome, it is still not entirely clear how the exosome core functions in concert with these two nucleases to degrade RNA. In this dissertation, I present structural and biochemical studies of both the Rrp44 and Rrp6 bound exosome. In particular, I have attempted to crystallize a sub-complex of Rrp44 bound to the exosome, and present data that suggest that structured RNA can access Rrp44 without being first channeled through the exosome. Despite this conclusion, the exosome core still appears to down-regulate both the exonuclease and endonuclease activities of Rrp44, possibly by causing the protein to assume a conformation that prohibits it from promiscuous RNA degradation. Work on the Rrp6 containing exosme was conducted in attempts of obtaining an EM reconstruction of this complex. Crystallization of an RNA and Rrp6 binding exosome associated iii protein, Rrp47, was also attempted alone and in complex with Rrp6. Overall, the results that I present here will help to further characterize the function of the exosome in RNA mediated degradation. i