Interactions crucial for three-dimensional domain swapping in the HP-RNase variant PM8

9 pags, 4 figs, 1 tabThe structural determinants that are responsible for the formation of higher order associations of folded proteins remain unknown. We have investigated the role on the dimerization process of different residues of a domain-swapped dimer human pancreatic ribonuclease variant. This variant is a good model to study the dimerization and swapping processes because dimer and monomer forms interconvert, are easily isolated, and only one dimeric species is produced. Thus, simple models for the swapping process can be proposed. The dimerization (dissociation constant) and swapping propensity have been studied using different variants with changes in residues that belong to different putative molecular determinants of dimerization. Using NMR spectroscopy, we show that these mutations do not substantially alter the overall conformation and flexibility, but affect the residue level stability. Overall, the most critical residues for the swapping process are those of one subunit that interact with the hinge loop of another one-subunit residue, stabilizing it in a conformation that favors the interchange. Tyr25, Gln101, and Pro 19, with Asn17, Ser21, and Ser23, are found to be the most significant; notably, Glu103 and Arg 104, which were postulated to form salt bridges that would stabilize the dimer, are not critical for dimerization. © 2011 Biophysical Society.Work was supported by grants No. BFU2009-06935/BMC, No. CTQ2010-21567-C02-02, and No. CTQ2008-00080/BQU from Ministerio de Ciencia e Innovación (Spain)