X-ray crystallographic structure of the light-harvesting biliprotein C-phycocyanin from the thermophilic cyanobacterium Mastigocladus laminosus and its resemblance to globin structures

The structure of the biliprotein C-phycocyanin from the thermophilic cyanobacterium Mastigocladus laminosus has been determined at 3 A resolution by X-ray diffraction methods. Phases have been obtained by the multiple isomorphous replacement method. The electron density map could be improved by solvent flattening and has been interpreted in terms of the amino acid sequence. The protein consists of three identical (alpha-beta)-units which are arranged around a threefold symmetry axis to form a disc of approximate dimensions 110 A X 30 A with a central channel of 35 A in diameter. This aggregation form is supposed to be the same as that found in the rods of native phycobilisomes. Both subunits, alpha and beta, exhibit a similar structure and are related by a local twofold rotational axis. Each subunit is folded into eight helices and irregular loops. Six helices are arranged to form a globular part, whereas two helices stick out and mediate extensive contact between the subunits. The arrangement of the helices of the globular part resembles the globin fold: 59 equivalent C alpha-atoms have a root-mean-square deviation of 2 X 9 A. The chromophores attached to cystein 84 of the alpha- and beta-subunits are topologically equivalent to the haem. All three chromophores of C-phycocyanin, open-chain tetrapyrroles, are in an extended conformation. alpha 84 and beta 84 are attached to helix E (globin nomenclature), beta 155 is linked to the G--H loop. The shortest centre-to-centre distance between chromophores in trimer is 22 A.