Structural basis for the excision repair of alkylation-damaged DNA

Base-excision DNA repair proteins that target alkylation damage act on a variety of seemingly dissimilar adducts, yet fail to recognize other closely related lesions. The 1.8 Angstrom crystal structure of the monofunctional DNA glycosylase AlkA (E. coli 9-methyladenine-DNA glycosylase II) reveals a large hydrophobic cleft unusually rich in aromatic residues. An Asp residue projecting into this cleft is essential for catalysis, and it governs binding specificity for mechanism-based inhibitors. We propose that AlkA recognizes electron-deficient methylated bases through pi-donor/acceptor interactions involving the electron-rich aromatic cleft. Remarkably, AlkA is similar in fold and active site location to the bifunctional glycosylase/lyase endonuclease III, suggesting the two may employ fundamentally related mechanisms for base excisionclos

Unfolding of Green Fluorescent Protein mut2 in wet nanoporous silica gels

Many of the effects exerted on protein structure, stability, and dynamics by molecular crowding and confinement in the cellular environment can be mimicked by encapsulation in polymeric matrices. We have compared the stability and unfolding kinetics of a highly fluorescent mutant of Green Fluorescent Protein, GFPmut2, in solution and in wet, nanoporous silica gels. In the absence of denaturant, encapsulation does not induce any observable change in the circular dichroism and fluorescence emission spectra of GFPmut2. In solution, the unfolding induced by guanidinium chloride is well described by a thermodynamic and kinetic two-state process. In the gel, biphasic unfolding kinetics reveal that at least two alternative conformations of the native protein are significantly populated. The relative rates for the unfolding of each conformer differ by almost two orders of magnitude. The slower rate, once extrapolated to native solvent conditions, superimposes to that of the single unfolding phase observed in solution. Differences in the dependence on denaturant concentration are consistent with restrictions opposed by the gel to possibly expanded transition states and to the conformational entropy of the denatured ensemble. The observed behavior highlights the significance of investigating protein function and stability in different environments to uncover structural and dynamic properties that can escape detection in dilute solution, but might be relevant for proteins in vivo