Trimeric domain-swapped barnase

The structure of a trimeric domain-swapped form of barnase (EC 3.1.27.3) wa s determined by x-ray crystallography at a resolution of 2.2 Angstrom from crystals of space group R32. Residues 1-36 of one molecule associate with r esidues 41-110 from another molecule related through three-fold symmetry. T he resulting cyclic trimer contains three protein folds that are very simil ar to those in monomeric barnase. Both swapped domains contain a nucleation site for folding. The formation of a domain-swapped trimer is consistent w ith the description of the folding process of monomeric barnase as the form ation and subsequent association of two foldons.