Solution structure and backbone dynamics of the human DNA ligase III alphaBRCT domain

(BRCA1 carboxyl terminus) domains are found in a number of DNA repair enzym es and cell cycle regulators and are believed to mediate important protein- protein interactions. The DNA ligase III alpha BRCT domain partners with th e distal BRCT domain of the DNA repair protein XRCC1 (X1BRCTb) in the DNA b ase excision repair (BER) pathway. To elucidate the mechanisms by which the se two domains can interact, we have determined the solution structure of h uman ligase III alpha BRCT (L3[86], residues 837-922). The structure of L3[ 86] consists of a beta2 beta1 beta3 beta4 parallel sheet with a two-a-helix bundle packed against one face of the sheet. This fold is conserved in sev eral proteins having a wide range of activities, including X1BRCTb [Zhang, X. D., et al. (1998) EMBO J. 17, 6404-6411]. L3[86] exists as a dimer in so lution, but an insufficient number of NOE restraints precluded the determin ation of the homodimer structure. However, C-13 isotope-filtered and hydrog en-deuterium exchange experiments indicate that the N-terminus, alpha1, the alpha1-beta2 loop, and the three residues following alpha2 are involved in forming the dimer interface, as similarly observed in the structure of X1B RCTb. NOE and dynamic data indicate that several residues (837-844) in the N-terminal region appear to interconvert between helix and random coil conf ormations. Further studies of other BRCT domains and of their complexes are needed to address how these proteins interact with one another, and to she d light on how mutations can lead to disruption of function and ultimately disease.