The Crystal Structure of Allophycocyanin and Its Comparison to C-Phycocyanin and b-Phycoerythrin

Abstract

The phycobiliprotein allophycocyanin (APC¹)from the cyanobacterium Spirulina platensis has been isolated and crystallised. The three-dimensional structure of the (αß) monomer has been solved by multiple isomorphous replacement techniques. The molecular structure of APC is similar to other known phycobiliproteins. In comparison to C-phycocyanin (C-PC) and b-phycoerythrin (b-PE) the major differences arise from deletions and insertions of segments involved in protein-chromophore interactions. In C-PC the absorption spectra of the monomers and the trimers do not differ markedly (near 620 nm). In APC the monomer spectrum is very similar to that of monomeric C-PC, but the spectrum of the trimer is shifted to 650 nm. The exciton interactions between the neighbouring pigments or changed chromophore conformation were proposed as the cause for the 650 nm absorption maximum in APC trimers. The comparison of the chromophore binding regions of both proteins indicates that a significant conformational change occurs in the α84 chromophore of APC due to the different protein environment of the α and neighbouring ß-subunit.