The dual-mode quaternary structure of seminal RNase

Bovine seminal ribonuclease, the only dimeric ribonuclease described thus far, is found to exist in two different quaternary structure forms. In one, the N-terminal segment (residues 1-17) of each subunit is interchanged with the remaining segment of the other subunit, whereas in the second, such interchange does not occur. Functionally, they differ in that the catalytic activity of the form with interchange can be modulated by the substrate, whereas the noninterchange form exhibits no cooperativity. Each form can convert into the other, up to an equilibrium ratio, which is that found for the isolated protein. The results of refolding experiments of unfolded protein chains suggest that also in vivo the form lacking interchange may be produced first and is then partially transformed into the other dimeric form until equilibrium is reached. Although the implications of these findings may not be immediately apparent, they are intriguing and may have an impact on the unusual noncatalytic actions of the protein, such as its selective cytotoxicity toward tumor cells, activated T cells, and differentiated male germ cells.