Abstact

The interactions necessary for stabilizing the folding of the N-terminal large beta-subdomain and the C-terminal small alpha-subdomain of staphylococcal nuclease (SNase) were investigated by an approach of fragment complementation. Two SNase fragments, namely, SNase121 and SNase(111-143) containing 1-121 and 111-143 residues, respectively, of native SNase, were used in this study since the sequences of the two fragments correspond to that of the beta- and alpha-subdomains of SNase. SNase121 is a largely unfolded fragment whereas SNase(111-143) is a structureless fragment. The recognition process and efficiency of complementation of SNase121 and SNase(111-143) fragments were studied by NMR and various biochemical and biophysical methods. SNase121 and SNase(111-143) can recognize each other and recover their native conformations on binding, restoring the active site and the ability to degrade DNA. The SNase121:SNase(111-143) complex showed a nuclease activity up to 30% that of native SNase. The final rigid structures of SNase121 and SNase(111-143) fragments having the folded native-like beta-subdomain and alpha-subdomain structures of SNase, respectively, in the complex form simultaneously with the complex stabilization. Studies with the mutant SNase121 and SNase(111-143) fragments reveal that the sequence elements which are essential for recognition and efficient complementation of the two fragments are also necessary for recovering the native-like interactions at the binding interface between them. The interfragment interactions that induce the structural complementation of SNase121 and SNase(111-143) likely reflect the tertiary interactions necessary to stabilize the folding of both beta- and alpha-subdomains in the native SNase.