Structures of an unliganded neurophysin and its vasopressin complex: Implications for binding and allosteric mechanisms

The structures of des 1-6 bovine neurophysin-II in the unliganded state and as its complex with lysine vasopressin were determined crystallographicall y at resolutions of 2.4 Angstrom and 2.3 Angstrom, respectively. The struct ure of the protein component of the vasopressin complex was, with some loca l differences, similar to that determined earlier of the full-length protei n complexed with oxytocin, but relatively large differences, probably intri nsic to the hormones, were observed between the structures of bound oxytoci n and bound vasopressin at Gln 4. The structure of the unliganded protein i s the first structure of an unliganded neurophysin. Comparison with the lig anded state indicated significant binding-induced conformational changes th at were the largest in the loop region comprising residues 50-58 and in the 7-10 region. A subtle binding-induced tightening of the subunit interface of the dimer also was shown, consistent with a role for interface changes i n neurophysin allosteric mechanism, but one that is probably not predominan t. Interface changes are suggested to be communicated from the binding site through the strands of beta -sheet that connect these two regions, in part with mediation by Gly 23. Comparison of unliganded and liganded states add itionally reveals that the binding site for the hormone alpha -amino group is largely preformed and accessible in the unliganded state, suggesting tha t it represents the initial site of hormone protein recognition. The potent ial molecular basis for its thermodynamic contribution to binding is discus sed.