The interconversion of isoforms of seminal ribonuclease: Modelling key intermediates and trypsin effects

The stepwise tryptic degradation of the interconverting quaternary isoforms of seminal ribonuclease has been analysed by structural modelling, based o n the experimental results obtained by treating the dimeric protein with tr ypsin. The results of the analysis were compared with those obtained applyi ng to the action of trypsin on seminal ribonuclease a recently proposed pre dictive algorithm for limited proteolysis. The attention was focussed on th e MxM form of the protein, in which the two subunits swap their N-terminal ends interconverting at equilibrium with the M=M form with no interchange b etween subunits. The analysis led to the identification of a key intermedia te in the interconversion pathway, and to the resolution of the apparent co ntradiction between prediction and actual experimental data. (C) 2000 Acade mic Press.