Lhcb9-dependent photosystem I structure in moss reveals evolutionary adaptation to changing light conditions during aquatic-terrestrial transition

Abstract

AbstractIn plants and green algae, light-harvesting complexes I and II (LHCI and LHCII) constitute the antennae of photosystem I (PSI), thus effectively increasing the cross-section of the PSI core. The mossPhyscomitrium patens(P. patens) represents a well-studied evolutionary intermediate between green algae and flowering plants.P. patenspossesses at least three types of PSI with different antenna sizes. The largest PSI form (PpPSI-L) exhibits a unique organization found neither in flowering plants nor in algae. Its formation is mediated by theP. patens-specific LHC protein, Lhcb9. While previous studies revealed the overall architecture of thePpPSI-L, its assembly details and the relationship between differentPpPSI types remain unclear. Here, we report a high-resolution structure of thePpPSI-L. We identified 14 PSI core subunits, one Lhcb9, one phosphorylated LHCII trimer, and eight LHCI monomers arranged as two belts. Our structural analysis established the essential role of Lhcb9 and the phosphorylated LHCII in stabilizing the complex. In addition, our results suggest thatPpPSI switches between three different types, which share identical modules. This feature may contribute to the quick and dynamic adjustment of the light-harvesting capability of PSI under different light conditions.