Structure of a unique PSII-Pcb tetrameric megacomplex in a chlorophyll <i>d</i>-containing cyanobacterium.

Abstract

<i>Acaryochloris marina</i> is a unique cyanobacterium using chlorophyll <i>d</i> (Chl <i>d</i>) as its major pigment and thus can use far-red light for photosynthesis. Photosystem II (PSII) of <i>A. marina</i> associates with a number of prochlorophyte Chl-binding (Pcb) proteins to act as the light-harvesting system. We report here the cryo-electron microscopic structure of a PSII-Pcb megacomplex from <i>A. marina</i> at a 3.6-angstrom overall resolution and a 3.3-angstrom local resolution. The megacomplex is organized as a tetramer consisting of two PSII core dimers flanked by sixteen symmetrically related Pcb proteins, with a total molecular weight of 1.9 megadaltons. The structure reveals the detailed organization of PSII core consisting of 15 known protein subunits and an unknown subunit, the assembly of 4 Pcb antennas within each PSII monomer, and possible pathways of energy transfer within the megacomplex, providing deep insights into energy transfer and dissipation mechanisms within the PSII-Pcb megacomplex involved in far-red light utilization.