STRUCTURE OF A FUNCTIONAL FRAGMENT OF VCAM-1 REFINED AT 1.9 ANGSTROM RESOLUTION

The crystal structure of the functional amino-terminal two-domain frag ment of human vascular cell adhesion molecule 1 (VCAM-1) has been dete rmined at 1.9 Angstrom resolution. The crystals contain two copies of the molecule in the asymmetric unit. The structure was solved by multi ple isomorphous replacement, using lead and selenium derivatives. Anom alous scattering had to be used to resolve the phase ambiguity of a le ad derivative. Since the selenium derivative has very small isomorphou s differences, the local scaling algorithm had to be used to obtain an interpretable difference Patterson map. The initial phases were impro ved by non-crystallographic averaging, solvent flattening and histogra m matching. The structure has been refined to a crystallographic R fac tor of 20.4% (15-1.9 Angstrom, F greater than or equal to 3 sigma) and consists of two Ig domains (D1 and D2). The angle between these domai ns differs by 12 degrees between the two copies of the molecule in the crystallographic asymmetric unit, demonstrating that some movement is possible at the interface. In the amino-terminal domain D1 there is a n 'extra' disulfide bond, in addition to the conserved cross-sheet dis ulfide bond, at the top of the molecule. This bond, a hallmark of the integrin-binding subclass of Ig superfamily proteins, makes the top of this domain very compact. The feature that projects most prominently from D1 is the CD loop, near the base of the domain. The key residue f or integrin binding, Asp40, is located in this loop and is easily acce ssible.