Molecular characteristics of recombinant human CD4 as deduced from polymorphic crystals.

We have grown crystals of a soluble recombinant form of human CD4, a transmembrane glycoprotein found predominantly on the surface of helper T cells. Crystals composed of the entire extracellular portion of CD4 exhibit extensive polymorphism. Of the five crystal types that have been grown, the best diffracts to Bragg spacings of 4.9 A. Symmetry considerations and characterization of the asymmetric unit by volume-specific amino acid analysis lead to the suggestion that a tetramer is the fundamental unit of crystallization. The characterization also showed that several of the crystal types have unusually high solvent contents. Because high solvent content and weak diffraction are indicative of an extended flexible structure, we examined the molecular shape of the recombinant CD4 with ultracentrifugation and found that it has an axial ratio of roughly 6, when modeled as a prolate ellipsoid. These results, combined with crystal packing constraints, suggest dimensions of approximately 25 x 25 x 125 A for a monomer. The structural features deduced here may be relevant to the biological function of CD4 as a receptor mediating cell-cell and cell-virus interactions.