The C-1 subunit of alpha-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
Ej. Gordon et al., The C-1 subunit of alpha-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, ACT CRYST D, 57, 2001, pp. 1230-1237
The previously unknown crystal structure of the C-1 subunit of the caroteno
id-binding protein alpha -crustacyanin has been determined using the anomal
ous scattering available at 1.77 Angstrom wavelength to determine the parti
al structure of the S atoms intrinsic to the native protein. The resulting
'heavy-atom' phases, in conjunction with near-atomic resolution (d(min) = 1
.15 Angstrom) data, were then used to initiate successful structure determi
nation using a direct-methods approach. This is, to the authors' knowledge,
the first time such a small anomalous signal (similar to 1%) has been used
to aid the determination of a macromolecular structure. As well as the str
ucture itself, the methods used during data collection and those used in th
e elucidation of the sulfur 'heavy-atom' partial structure are described he
re. As predicted, the C-1 subunit adopts a tertiary structure typical of th
e lipocalin superfamily: an eight-stranded antiparallel beta -barrel with a
repeated +1 topology. The beta -barrel has a calyx shape with the two mole
cules in the asymmetric unit interacting in such a way that the open ends o
f 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 l
ipocalin superfamily has allowed speculation as to the nature of carotenoid
binding by the protein.