Structure of lobster apocrustacyanin A(1) using softer X-rays

The molecular basis of the camouflage colouration of marine crustacea is of ten provided by carotenoproteins. The blue colour of the lobster carapace, for example, is intricately associated with a multimacromolecular 16-mer co mplex of protein subunits each with a bound astaxanthin molecule. The prote in subunits of crustacyanin fall into two distinct subfamilies, CRTC and CR TA. Here, the crystal structure solution of the A(1) protein of the CRTC su bfamily is reported. The problematic nature of the structure solution of th e CRTC proteins (both C-1 and A(1)) warranted consideration and the develop ment of new approaches. Three putative disulfides per protein subunit were likely to exist based on molecular-homology modelling against known lipocal in protein structures. With two such subunits per crystallographic asymmetr ic unit, this direct approach was still difficult as it involved detecting a weak signal from these sulfurs and suggested the use of softer X-rays, co mbined with high data multiplicity, as reported previously [Chayen et al. ( 2000), Acta Cryst. D56, 1064-1066]. This paper now describes the structure solution of CRTC in the form of the A(1) dimer based on use of softer X-ray s (2 Angstrom wavelength). The structure solution involved a xenon derivati ve with an optimized xenon L-I edge f " signal and a native data set. The h and of the xenon SIROAS phases was determined by using the sulfur anomalous signal from a high-multiplicity native data set also recorded at 2 Angstro m wavelength. For refinement, a high-resolution data set was measured at sh ort wavelength. All four data sets were collected at 100 K. The refined str ucture to 1.4 Angstrom resolution based on 60 276 reflections has an R fact or of 17.7% and an R-free of 22.9% (3137 reflections). The structure is tha t of a typical lipocalin, being closely related to insecticyanin, to bilin- binding protein and to retinol-binding protein. This A(1) monomer or dimer can now be used as a search motif in the structural studies of the oligomer ic forms alpha- and beta -crustacyanins, which contain bound astaxanthin mo lecules.