Crystal Structure of the S. solfataricus Archaeal Exosome Reveals Conformational Flexibility in the RNA-Binding Ring

Background The exosome complex is an essential RNA 3′-end processing and degradation machinery. In archaeal organisms, the exosome consists of a catalytic ring and an RNA-binding ring, both of which were previously reported to assume three-fold symmetry. Methodology/Principal Findings Here we report an asymmetric 2.9 Å Sulfolobus solfataricus archaeal exosome structure in which the three-fold symmetry is broken due to combined rigid body and thermal motions mainly within the RNA-binding ring. Since increased conformational flexibility was also observed in the RNA-binding ring of the related bacterial PNPase, we speculate that this may reflect an evolutionarily conserved mechanism to accommodate diverse RNA substrates for degradation. Conclusion/Significance This study clearly shows the dynamic structures within the RNA-binding domains, which provides additional insights on mechanism of asymmetric RNA binding and processing.