A potential allosteric subsite generated by domain swapping in bovine seminal ribonuclease

Bovine seminal ribonuclease (BS-RNase) is a peculiar member of the pancreat ic-like ribonuclease superfamily endowed with unique biological functions. It has been shown that native BS-RNase is a mixture of two distinct dimeric forms. The most abundant form is characterised by the swapping of the N-te rminal helix. Kinetic studies have shown that this dimer is allosterically regulated, whereas the minor component, in which no swapping occurs, exhibi ts typical Michaelian kinetics, in order to correlate the catalytic propert ies with the structural features of BS-RNase, we have determined the crysta l structure of the BS-RNase swapping dimer complexed with uridylyl(2'-5') g uanosine. The structure of the complex was refined to an R value of 0.189 a t 1.9 Angstrom resolution. Surprisingly, the enzyme binds four dinucleotide molecules, all in a non-productive way. in the two active sites, the guani ne base is located in the subsite that is specific for pyrimidines. This un usual binding has been observed also in complexes of RNase A with guanine-c ontaining nucleotides (retro-binding). One of the two additional dinucleoti de molecules bound to the enzyme is located an the surface of the protein i n a pocket generated by crystal packing; the second was found in a cavity a t the interface between the two subunits of the swapping dimer. There are i ndications that the interface site plays a role in the allosteric regulatio n exhibited by BS-RNase. This finding suggests that domain swapping may not merely be a mechanism that proteins adopt for the transition from a monome ric to oligomeric state but can be used to achieve modulations in catalytic function. (C) 1999 Academic Press.