3D domain swapping and protein aggregation

Bovine pancreatic ribonuclease (RNase A) has been converted to a fibrillar form by expanding, with an insertion of ten glutamines, the peptide connecting the core domain of the protein with the C-terminal b-strand [1]. The designed amyloid-like fiber of RNase A contains 3D domain swapped molecules capable of enzymatic activity, thus supporting the amyloid-like fiber model in which a cross-b spine is decorated with 3D domain swapping. By deleting five aminoacid residues in the hinge loop linking the N-terminal helix of human pancreatic RNase (HP-RNase) to the rest of the protein, we found that HP-RNase is able to form domain swapped dimers. These dimers cannot be dissociated by heating at 70 degrees C, and have a great tendency to aggregate. X-ray structure analysis of the HP-RNase dimer reveals that the molecules give rise to a double-helical arrangement that is stabilized by the exchange of N-terminal fragments between subunits belonging to different helices. The observed supra-molecular assembly suggests that the cross b-spine could be not the only way to produce fibres.