Structure and assembly mechanism of plant C₂S₂M₂-type PSII-LHCII supercomplex

In plants, the photosynthetic machinery photosystem II (PSII) consists of a core complex associated with variable numbers of light-harvesting complexes II (LHCIIs). The supercomplex, comprising a dimeric core and two strongly bound and two moderately bound LHCIIs (C₂S₂M₂), is the dominant form in plants acclimated to limited light. Here we report cryo-electron microscopy structures of two forms of C₂S₂M₂ (termed stacked and unstacked) from Pisum sativum at 2.7- and 3.2-angstrom resolution, respectively. In each C₂S₂M₂, the moderately bound LHCII assembles specifically with a peripheral antenna complex CP24-CP29 heterodimer and the strongly bound LHCII, to establish a pigment network that facilitates light harvesting at the periphery and energy transfer into the core. The high mobility of peripheral antennae, including the moderately bound LHCII and CP24, provides insights into functional regulation of plant PSII.