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 A 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.
