STUDY OF THE INTERACTIONS BETWEEN NEUROPHYSIN-II AND DIPEPTIDE LIGANDBY MEANS OF MOLECULAR-DYNAMICS

The nonapeptide hormones oxytocin (OT) and vasopressin (VP), while tra nsported in the posterior pituitary, are packaged into neurosecretory granules (NSG) in the form of high associates with disulfide-rich prot eins known as neurophysin I(NPI) and neurophysin II (NPII), respective ly. In the NSG, neurophysins serve as carrier proteins to the hormones , until the latter are dissociated upon secretion into blood. To shed more light on molecular self-recognition between NPs, and between NPs and their ligands, we have studied their molecular association, using as a starting point the recently published solid-state structure (C-al pha-trace) of the neurophysin II-dipeptide complex. Another purpose of this work was the development of reliable strategies for molecular mo deling, that would utilize minimal structural information (like C-alph a trace and/or structural homology) yet be useful for studies of prote in/ligand interactions. An initial all-atom representation of the prot ein-peptide complex (2:2) was obtained py the conversion of the C-alph a-carbon trace deposited in the Brookhaven Protein Data Bank (file 1BN 2), using the InsightII/Biopolymer modules from the suite of programs supplied by Biosym Technologies, San Diego. The free NPII homodimer wa s obtained by removal of the dipeptide ligands from the starting struc tures. Both associates, after initial immersion in water, were submitt ed to gradual (side chains first then all atoms) minimization of energ y. Subsequently, they were thermally equilibrated and submitted to the molecular dynamics (AMBER 4.0) at 300K, until the total energy was st abilized. The structures, averaged over the last 20 ps of the dynamics , were compared with the starting C-alpha-trace and among themselves. The protein/ligand complex, simulated in water, compares favourably wi th the solid-state reference. An allosteric mechanism for the NPII dim er/ligand interaction is proposed and discussed.