The C₁ subunit of α-crustacyanin: the de novo phasing of the crystal structure of a 40 kDa homodimeric protein using the anomalous scattering from S atoms combined with direct methods

The previously unknown crystal structure of the C₁ subunit of the carotenoid-binding protein α-crustacyanin has been determined using the anomalous scattering available at 1.77 Å wavelength to determine the partial structure of the S atoms intrinsic to the native protein. The resulting `heavy-atom' phases, in conjunction with near-atomic resolution (d<sub>min</sub> = 1.15 Å) data, were then used to initiate successful structure determination using a direct-methods approach. This is, to the authors' knowledge, the first time such a small anomalous signal (∼1%) has been used to aid the determination of a macromolecular structure. As well as the structure itself, the methods used during data collection and those used in the elucidation of the sulfur `heavy-atom' partial structure are described here. As predicted, the C₁ subunit adopts a tertiary structure typical of the lipocalin superfamily: an eight-stranded antiparallel β-barrel with a repeated +1 topology. The β-barrel has a calyx shape with the two molecules in the asymmetric unit interacting in such a way that the open ends of each calyx face each other, although they do not form a single elongated pocket. A comparison of this structure with those of other members of the lipocalin superfamily has allowed speculation as to the nature of carotenoid binding by the protein.