THE STRUCTURE OF THE AMINO-TERMINAL IG-LIKE SIALIC ACID BINDING DOMAIN OF SIALOADHESIN. A.May1, R.C.Robinson1, P.Bradfield2, M.Vinson2, P.R.Crocker2, E.Y.Jones1. 1Laboratory of Molecular Biophysics, University of Oxford, Oxford. U.K; 2I.C.R.F. Laboratories, University of Oxford, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford. U.K.
A functional fragment of Sialoadhesin, which recognises oligosaccharides terminating in NeuAc[[alpha]]2-3Gal in N- and O-linked glycans, has been crystallised, and its structure has been solved to 2.6Å resolution using MAD. Sialoadhesin is a macrophage-restricted receptor containing 17 Immunoglobulin(Ig)-like domains, of which the N-terminal domain is necessary and sufficient to mediate sialic-acid dependent binding (1). The structure consists of a single V-set Ig domain, containing 115 amino acids. The characteristic Ig inter sheet disulphide bridge is replaced by an intra-sheet disulphide between the B and E strands. The structure most closely resembles a monomer of CD8[[alpha]], with loops surrounding the binding site identified by site-directed mutagenesis (2). Residues implicated in sialic acid binding are found on the G-F-C-C'-C" face. This face also forms the interactive surface in CD2 and VCAM-1, other cell surface members of the IgSF. For MAD phasing, data sets were collected at three wavelengths from a single crystal flash-frozen at 104K. The crystals belong to space group P3121 with unit cell dimensions of a = b = 38.9, c = 152.6, [[alpha]] = [[beta]] = 90deg., [[gamma]] = 120deg., with one molecule in the crystallographic asymmetric unit.
(1) Nath., D., van der Merwe, P.A., Kelm, S., Bradfield, P. and Crocker, P.R. (1995) J.Biol.Chem. vol.270 no.44 pp.26184-26191
(2) Vinson, M., van der Merwe, P.A., Kelm, S., May, A., Jones, E.Y., and Crocker, P.R., (1996) J.Biol.Chem. (in press)