Metal-mediated self-assembly of a beta-sandwich protein.

The beta-sandwich cupredoxin Plastocyanin (Pc) was found to self-assemble in the presence of Zn(2+), a known mediator of protein-protein interfaces. Diffraction-quality crystals of Pc grew from solutions containing zinc acetate as the sole precipitant. Di- and trinuclear zinc sites contribute to the crystal contacts in this structure. A different crystal form, also involving numerous zinc bridging ions, was obtained in the presence of poly(ethylene glycol) 8 000. Comparison of the two crystal forms reveals the effect of macromolecular crowding on self-assembly. Solution-state structural characterisation of the Zn(2+)-mediated Pc oligomers was performed by using a combination of chemical shift perturbation mapping and small-angle X-ray scattering. The data indicate the formation of dimers in solution. The implications for metal-mediated assembly and crystallisation are discussed.