Science Highlights: November 2011 Archives
Tsutakawa et al. have combined solution Small Angle X-ray Scattering (SAXS) data collected at the SIBYLS beamline with computational modeling carried out at Oak Ridge Leadership Computing Facility ( OLCF ) to elucidate new modes of flexibility in a key protein complex (Ubiquitin-PCNA) involved in DNA replication and repair. The work was published in the Oct 25th issue of PNAS, and has been highlighted by OLCF and the Faculty of 1000.
PCNA ubiquitination in response to DNA damage leads to the recruitment of specialized translesion polymerases to the damage locus. This constitutes one of the initial steps in translesion synthesis (TLS)-a critical pathway for cell survival and for maintenance of genome stability. The recent crystal structure of ubiquitinated PCNA (Ub-PCNA) sheds light on the mode of association between the two proteins but also revealed that paradoxically, the ubiquitin surface engaged in PCNA interactions was the same as the surface implicated in translesion polymerase binding. This finding implied a degree of flexibility inherent in the Ub-PCNA complex that would allow it to transition into a conformation competent to bind the TLS polymerase. To address the issue of segmental flexibility, we combined multiscale computational modeling and small angle X-ray scattering. This combined strategy revealed alternative positions for ubiquitin to reside on the surface of the PCNA homotrimer, distinct from the position identified in the crystal structure. Two mutations originally identified in genetic screens and known to interfere with TLS are positioned directly beneath the bound ubiquitin in the alternative models. These computationally derived positions, in an ensemble with the crystallographic and flexible positions, provided the best fit to the solution scattering, indicating that ubiquitin dynamically associated with PCNA and is capable of transitioning between a few discrete sites on the PCNA surface. The finding of new docking sites and the positional equilibrium of PCNA-Ub occurring in solution provide unexpected insight into previously unexplained biological observations
SAXS data collected at the SIBYLS beamline was used in conjunction with high resolution crystals structures to discern details of the unique interaction mode of of these key players in the autophagy pathway.
Atg7 is a noncanonical, homodimeric E1 enzyme that interacts with the noncanonical E2 enzyme, Atg3, to mediate conjugation of the ubiquitin-like protein (UBL) Atg8 during autophagy. Here we report that the unique N-terminal domain of Atg7 (Atg7NTD) recruits a unique “flexible region” from Atg3 (Atg3FR). The structure of an Atg7NTD-Atg3FR complex reveals hydrophobic residues from Atg3 engaging a conserved groove in Atg7, important for Atg8 conjugation. We also report the structure of the homodimeric Atg7 C-terminal domain, which is homologous to canonical E1s and bacterial antecedents. The structures, SAXS, and crosslinking data allow modeling of a full-length, dimeric (Atg7∼Atg8-Atg3)2 complex. The model and biochemical data provide a rationale for Atg7 dimerization: Atg8 is transferred in trans from the catalytic cysteine of one Atg7 protomer to Atg3 bound to the N-terminal domain of the opposite Atg7 protomer within the homodimer. The studies reveal a distinctive E1∼UBL-E2 architecture for enzymes mediating autophagy.
Using data collected at the SIBYLS beamline members of the Beese Lab have demonstrated that high fidelity polymerases can stabilize certain tautomeric forms of mismatched base pairs ultimately resulting in incorporation of mutations into the DNA duplex.
Even though high-fidelity polymerases copy DNA with remarkable accuracy, some base-pair mismatches are incorporated at low frequency, leading to spontaneous mutagenesis. Using high-resolution X-ray crystallographic analysis of a DNA polymerase that catalyzes replication in crystals, The Beese Lab has observed that a C•A mismatch can mimic the shape of cognate base pairs at the site of incorporation. This shape mimicry enables the mismatch to evade the error detection mechanisms of the polymerase, which would normally either prevent mismatch incorporation or promote its nucleolytic excision. Movement of a single proton on one of the mismatched bases alters the hydrogen-bonding pattern such that a base pair forms with an overall shape that is virtually indistinguishable from a canonical, Watson-Crick base pair in double-stranded DNA. These observations provide structural evidence for the rare tautomer hypothesis of spontaneous mutagenesis, a long-standing concept that has been difficult to demonstrate directly.