Heparin-aggregated RANTES can be crystallised

Crystals of RANTES (regulated on activation normal T-cell expressed) have been grown in the presence of heparin-derived sulphated oligosaccharides which cause RANTES to aggregate severely. The crystals have a tendency to be polycrystalline but diffract to 3.8 - 1.8 A resolution. Oligosaccharides length and stoichiometry influence aggregation, nucleation and crystal growth and quality. Surprising, rather than inhibiting crystallisation, aggregation appears to stimulate nucleation. We have co-crystallised RANTES in the presence of oligosaccharides ranging in size from 1 to 12 sugar moieties. The best crystals, both according to size and diffraction quality have been obtained with a six moiety sugar. Crystals grow in the same space group with similar cell parameters as previously reported. We find no homogeneous binding of the sulphated sugars to RANTES even after eliminating sulphate from the crystallisation conditions to avoid competition with sulphates from the sulphated sugars. There is no electron density at the sulphate positions characterised in the original structure and the residues involved in sulphate binding adopt a different side chain orientation. Either heterogeneous binding of the sulphated sugars or an active ratchet-like mechanism by which the sulphated sugars are eliminated from the crystal lattice as the crystals grow may be responsible for the absence of sugars in the structure. The fact that aggregated proteins can be crystallised is important, since it is generally accepted that proteins that are polydisperse by light dynamic scattering are poor candidates for crystallisation trials.