Selective and asymmetric action of trypsin on the dimeric forms of seminalRNase

Dimeric seminal RNase (BS-RNase) is an equilibrium mixture of conformationa lly different quaternary structures, one characterized by the interchange b etween subunits of their N-terminal ends (the MXM form); the other with no interchange (the M=M form). Controlled tryptic digestion of each isolated q uaternary form generates, as limit digest products, folded and enzymaticall y active molecules, very resistant to further tryptic degradation. Electros pray mass spectrometric analyses and N-terminal sequence determinations ind icate that trypsin can discriminate between the conformationally different quaternary structures of seminal RNase, and exerts a differential and asymm etric action on the two dimeric forms, depending on the original quaternary conformation of each form. The two digestion products from the MXM and the M=M dimeric forms have different structures, which are reminiscent of the original quaternary conformation of the dimers: one with interchange, the o ther with no interchange, of the N-terminal ends. The surprising resistance of these tryptic products to further tryptic action is explained by the pe rsistence in each digestion product of the original intersubunit interface.