ELUCIDATION OF NEUROPHYSIN BIOLIGAND INTERACTIONS FROM MOLECULAR MODELING/

This is a review of our recent modeling work aimed at: (i) development and assessment of techniques for reliable refinement of low-resolutio n protein structures and (ii) using these techniques, at solving speci fic problems pertinent to neurophysin-bioligand interactions. Neurophy sins I and II (NPI and NPII) serve in the neurosecretory granules of t he posterior pituitary as carrier proteins for the neurophyseal hormon es oxytocin (OT) and vasopressin (VP), respectively, until the latter are released into blood. NPs are homologous two-domain, sulphur rich s mall proteins (93-95 residues, 7 disulphide bridges per monomer), capa ble of being aggregated. The Ca symmetrical NPI2 and NPII2 homodimers, and the (NPI/OT)(2) and (NPII/VP)(2) heterotetramers, all believed to be the smallest functional units, were modeled using low-resolution s tructure information, i.e. the C-alpha-carbon coordinates of the homol ogous NPII/dipeptide complex as a template. The all-atom representatio ns of the models were obtained using the SYBYL suite of programs (by T ripes, Inc.). Subsequently, they were relaxed, using a constrained sim ulated annealing (CSA) protocol, and submitted to about 100 ps molecul ar dynamics (MD) in water, using the AMBER 4.1 force field. The (NPI/O T)(2) and (NPII/VP)(2) structures, averaged after the last 20 ps of MD , were remarkably similar to those recently reported either for NPII/d ipeptide or NPII/oxytocin complex in the solid state (Chen ct al., 199 1, Proc. Natl. Acad. Sci., U.S.A. 88, 4240-4244; Rose et al., 1996, Na ture Struct. Biol. 3, 163-169). The results indicate that the 3(10) he lices (terminating the amino domains) and the carboxyl domains are mor e mobile than the remainder of the NP monomers. The hormones become an chored by residues 1-3 and 6 to the host, leaving residues 4-5 and 7-9 exposed on the surface and free to move. A cluster of attract ive-int eractions, extending from the ligand binding site, Tyr-24-Ile-26 of un it 1(2), to the inter-monomer interface Val-36 of unit 1(2), Cys-79 an d Ile-72 of unit 2(1), is clearly seen. We suggest that both these int eractions as well as the increased mobility of the 3(10) helix and the carboxyl domain may contribute to the allosteric communication betwee n the ligand and the unit1-unit2 interface.