Molecular structure, localization and function of biliproteins in the chlorophyll a/d containing oxygenic photosynthetic prokaryote Acaryochloris marina

We investigated the localization, structure and function of the biliprotein s of the oxygenic photosynthetic prokaryote Acaryochloris marina, the sole organism known to date that contains chlorophyll d as the predominant photo synthetic pigment. The biliproteins were isolated by means of sucrose gradi ent centrifugation, ion exchange and gel filtration chromatography. Up to s ix biliprotein subunits in a molecular mass range of 15.5-18.4 kDa were fou nd that cross-reacted with antibodies raised against phycocyanin or allophy cocyanin from a red alga. N-Terminal sequences of the alpha- and beta-subun its of phycocyanin showed high homogeneity to those of cyanobacteria and re d algae, but not to those of cryptomonads. As shown by electron microscopy, the native biliprotein aggregates are organized as rod-shaped structures a nd located on the cytoplasmic side of the thylakoid membranes predominantly in unstacked thylakoid regions. Biochemical and spectroscopic analysis rev ealed that they consist of four hexameric units, some of which are composed of phycocyanin alone, others of phycocyanin together with allophycocyanin. Spectroscopic analysis of isolated photosynthetic reaction center complexe s demonstrated that the biliproteins are physically attached to the photosy stem II complexes, transferring light energy to the photosystem II reaction center chlorophyll d with high efficiency. (C) 1999 Elsevier Science B.V. All rights reserved.