Structural insights into photosystem II supercomplex and trimeric FCP antennae of a centric diatom Cyclotella meneghiniana

Abstract

Diatoms are dominant marine algae and contribute around a quarter of the global primary productivity. The ecological success of diatoms is largely attributed to their photosynthetic capacity due to the presence of specific fucoxanthin chlorophyll-binding proteins (FCPs) as antennae to enhance the absorption of blue-green light under water. We purified a PSII-FCPII supercomplex and a trimeric FCP from Cyclotella meneghiniana (Cm), and solved their structures by single particle cryo-electron microscope (cryo-EM). The structures showed detailed organizations of monomeric, dimeric and trimeric FCP antennae, as well as new assemblies of an Lhcx6_1 and dimeric FCP-H in the PSII core. In each Cm-PSII-FCPIImonomer, an Lhcx6_1, an FCP dimer and other three FCP monomers are bound, which form an efficient Chls a network to relay excitation energy. More diadinoxanthins and diatoxanthins are found in the FCPs, which may function to quench excess energy. The trimeric FCP contained more Chls c and fucoxanthins. These diversified FCPs and PSII-FCPII provide a structural basis for efficient light energy harvesting, transfer, and dissipation process in C. meneghiniana.