Abstract
Phycobilisome (PBS) is the main light-harvesting antenna in cyanobacteria and red algae. How PBS transfers the light energy to photosystem II (PSII) remains to be elucidated. Here we report the in situ structure of the PBS-PSII supercomplex from Porphyridium purpureum UTEX 2757 using cryo-electron tomography and subtomogram averaging. Our work reveals the organized network of hemiellipsoidal PBS with PSII on the thylakoid membrane in the native cellular environment. In the PBS-PSII supercomplex, each PBS interacts with six PSII monomers, of which four directly bind to the PBS, and two bind indirectly. Additional three 'connector' proteins also contribute to the connections between PBS and PSIIs. Two PsbO subunits from adjacent PSII dimers bind with each other, which may promote stabilization of the PBS-PSII supercomplex. By analyzing the interaction interface between PBS and PSII, we reveal that αLCM and ApcD connect with CP43 of PSII monomer and that αLCM also interacts with CP47' of the neighboring PSII monomer, suggesting the multiple light energy delivery pathways. The in situ structures illustrate the coupling pattern of PBS and PSII and the arrangement of the PBS-PSII supercomplex on the thylakoid, providing the near-native 3D structural information of the various energy transfer from PBS to PSII.
Highlights
Life on Earth depends on photosynthesis for the conversion of solar energy to chemical energy
The tomogram slice shows that most PBS–photosystem II (PSII) supercomplexes are packed in an orderly fashion on the thylakoid membranes (Figure 1—figure supplement 1A)
Using in situ cryo-ET, our work reports the native PBS–PSII supercomplex structure of red algae
Summary
Life on Earth depends on photosynthesis for the conversion of solar energy to chemical energy. The energy absorbed by Rod transfers unidirectionally from the distal PE to the basal PC, and funnels to APC in the core, and eventually to the two terminal emitters, including chromophores in the core–membrane linker protein (LCM, called ApcE) (Capuano et al, 1991; Lundell et al, 1981; Tang et al, 2015) and allophycocyanin D (ApcD) (Glazer and Bryant, 1975; Peng et al, 2014). By using this approach, we identified the native PBS–PSII supercomplex and the double PBS– PSII supercomplex containing two adjacent PBS–PSII supercomplexes at resolutions of 14.3 Å and 15.6 Å, respectively These results illustrate the coupling pattern of PBS and PSII and the arrangement of PBS–PSII supercomplexes on the ordered distribution region of the thylakoid membrane, which cannot be determined with other methods. The newly found structural information provides a better understanding of energy transfer from PBS to PSII
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