SOLUTION STRUCTURES OF THE FIBRONECTIN-LIKE LEISHMANIA GP63 SRYD-CONTAINING SEQUENCE IN THE FREE AND ANTIBODY-BOUND STATES - TRANSFERRED NOE AND MOLECULAR-DYNAMICS STUDIES

The anti-SRYD monoclonal antibody (mAbSRYD) raised against the IASRYDQ L synthetic octapeptide, the 250-257 sequence of the Leishmania major surface glycoprotein gp63 recognizes both SRYD-containing peptides and the whole cognate major surface protein on intact parasites. Two SRYD -containing peptides. which antigenically and functionally mimic the R GDS sequence of fibronectin and efficiently inhibit parasite attachmen t to the macrophage receptors. were studied by two-dimensional transfe rred nuclear Overhauser effect experiments in the presence of mAbSRYD. The antibody-bound IASRYDQL octapeptide solution conformation was det ermined on the basis of 55 interproton-distance restraints, derived fr om NMR measurements. Eighteen structures which were first generated us ing an approach combining distance geometry and molecular dynamics, co nverge by energy minimization toward a folded structure with an averag e rmsd from the experimental data of less than 0.05 nm for the overall backbone and 0.025 nm for the SRYD motif. A distorted y-turn was foun d, stabilized by the backbone-backbone D255-NH to R253-CO hydrogen bon d, while the R253 and D255 side chains are pointing in opposite direct ions. This latter antibody-bound structure is compared with that of th e free octapeptide in dimethylsulfoxide solution, and with thr crystal structure of the RYD fragment in OPG2 Fab, an antireceptor antibody t hat mimics the RGD cell adhesion site. On this basis, a mechanism for IASRYDQL-receptor interaction is discussed.