No abstract

The cyanobacteria and red algae are two important groups of photosynthetic organisms that share a light-harvesting antenna known as the phycobilisome (PBS) [1][2][3][4] . PBSs are among the largest protein complexes in the living world and consist of phycobiliproteins (PBPs), including phycocyanin, phycoerythrin and allophycocyanin (APC), and linker proteins . Two subunits of PBPs, the α -and β -subunits, form an α β heterodimer that is conventionally called an (α β ) monomer. The monomer is then assembled into an (α β ) 3 trimer, the basic unit of PBS hierarchical assembly. The trimers of various PBPs are organized into a highly ordered supramolecular complex with the help of the linker proteins 1,5,6 . Four morphological types of PBS 1 are known: hemidiscoidal 7 , hemiellipsoidal 8 , block-type 9 and bundle-type 10. The hemidiscoidal PBS contains a central core surrounded by peripheral rods 1,11,12 . The chromophores in hemidiscoidal PBS are arranged in such a way that a photon absorbed by a chromophore in the peripheral rods is rapidly funnelled to chromophores in the core 13 and eventually to the terminal emitters (the core-membrane linker protein (L CM ) [14][15][16] or allophycocyanin D (ApcD) 17,18 ). The terminal emitters then transfer the energy to photosynthetic reaction centres 16,[19][20][21][22][23] . Currently, the mechanism of PBS assembly is poorly understood and the energy transfer routes within PBSs are not well defined. Although 3D structures of some individual PBPs have been reported (reviewed in refs 2, 3), the structures of most linker proteins are unknown and the complete structure of a PBS has not been published, to our knowledge. Here we report the cryo-electron microscopy (cryo-EM) structure of a PBS from the red alga Griffithsia pacifica at a resolution of 3.5 Å, which reveals details of the PBS architecture. Overall structureThe PBS from G. pacifica was purified and its intactness confirmed by its protein composition and spectroscopic features (Extended Data Fig. 1a-f). We reconstructed a 3D structure of the intact PBS by single particle cryo-EM with an overall resolution of 3.5 Å (Extended Data Table 1). Applying individual local masks improved the resolutions of local maps to 3.4-4.3 Å (Extended Data Fig. 2g). The PBS is one of the largest supramolecular complexes that has been reported, with a calculated molecular mass of approximately 16.8 megadaltons. The overall appearance is block-type 9 with twofold symmetry oriented perpendicularly to the thylakoid membranes (Fig. 1a-c and Extended Data Figs 1g-l, 3a-d). This PBS is larger than the hemiellipsoidal PBS isolated from Porphyridium cruentum8 and has dimensions of approximately 680 Å length, 390 Å height, and 450 Å thickness (Fig. 1a-c).The PBS contains a triangular core with the top cylinder B (formed by two APC trimers) sitting above two basal cylinders A and A′ (each formed by three APC trimers) surrounded by peripheral rods arranged in a staggered fashion (Fig. 1a-c and Extended Data Fig. 3c-e). In addition to the core and ...

No abstract

No abstract

No abstract