Phycobilisome and Phycobiliprotein Structures

Summary

Phycobilisomes serve as the primary light-harvesting antennae for Photosystem II in cyanobacteria and red algae. These supramolecular complexes are primarily composed of phycobiliproteins, a brilliantly colored family of water-soluble proteins bearing covalently attached, open-chain tetrapyrroles known as phycobilins. In addition, phycobilisomes also contain smaller amounts ‘linker polypeptides,’ most of which do not bear chromophores. These components are absolutely required for proper assembly and functional organization of the structure. Phycobilisomes are constructed from two main structural elements: a core substructure and peripheral rods that are arranged in a hemidiscoidal fashion around that core. The core of most hemidiscoidal phycobilisomes is composed of three cylindrical subassemblies. The peripheral rods radiate from the lateral surfaces of the core substructure which are not in contact with the thylakoid membrane. Absorbed light energy is transferred by very rapid, radiation-less downhill energy transfer from phycoerythrin or phycoerythrocyanin (if present) to C-phycocyanin and then to allophycocyanin species that act as the final energy transmitters from the phycobilisome to the Photosystem II or (partially) Photosystem I reaction centers.

This chapter focuses on important recent developments concerning the structure and function of phycobilisome architecture and their constituent phycobiliproteins. Studies with the phycobilisomes and phycobiliproteins of the cyanobacterium Mastigocladus laminosus as will be emphasized. During the last decade tremendous progress has been made in the molecular biology of cyanobacteria—through gene sequencing, genetic analyses, and studies on gene regulation during adaptation. Many specialized functions of phycobilisome components have been revealed through the construction and characterization of deletion or insertional mutants and mutants harboring site-specific changes in phycobiliproteins. New phycobiliproteins, that play an important role in open-sea photosynthesis, have been discovered from marine cyanobacteria. Previously known to occur in red algae only, γsubunits have been discovered in these marine cyanobacteria. The complete amino acid sequences for this last class of phycobiliproteins have now been determined. The structures of new chromophores from cryptomonad phycobiliproteins have been determined. Enzymes catalyzing site-specific bilin-attachment to apo-biliproteins have been described and their genes sequenced. The amino acid sequences of all components constituting the phycobilisomes of some cyanobacterial strains have been determined, and analyses of these data have revealed phylogenetic relationships. Structural and functional domains of the linker polypeptides have been recognized, and the special roles of the multifunctional, large core-membrane linker phycobiliprotein in assembling the phycobilisome core and in energy transfer were discovered. The crystal structures for several phycobiliproteins have now been determined at near-atomic resolution. These studies provide not only the protein structures, but additionally provide the details of chromophore-protein interactions and the basis for understanding energy-transfer mechanisms and kinetics.