Linear Dichroism and Orientation of Chromophores in Nostoc sp. Phycobilisomes and Their Subunits

Abstract

Phycobilisomes (PBS) are supramolecular aggregates of phycobiliproteins (PB proteins), which harvest light energy and deliver this energy to the photosynthetic reaction centers in cyanobacteria. The Nostoc sp. hemidiscoidal phycobilisomes grown in cool white fluorescent light has six rods, each consisting of four disks of phycoerythrin (PE) stacked onto two disks of phycocyanin (PC). Each of these disks consists of a trimeric assembly of PB protein complexes. These six rod-like structures are attached to a core consisting of three parallel allophycocyanin (APC) cylinders so that their longitudinal axes are perpendicular to the axes of the APC cylinders. Each of the three APC core cylinders consists of four disk-like assemblies of trimeric APC protein complexes, and the three APC cylinders are stacked so that their ends form a triangle. Linker polypeptides hold the disks together and may also influence the transfer of excitation energy. The PBS are attached by means of a 95 kD colored linker polypeptide to the thylakoid membrane. There is considerable interest in deducing the structure-function relationships which allow for the highly efficient transfer of excitation energy in phycobilisomes. The PE pigments are located at the distal end of the rods, while the PC pigments are located closer to the APC core cylinders. It is well known that the energy transfer sequence involves the PE → PC → APC → chlorophyll a pathway. Recent X-ray studies of PC trimer crystals derived from M. laminosus [1] has allowed for a detailed appreciation of the influence of orientational and positional parameters on energy transfer pathways in these PB proteins [2].