Architecture of the chloroplast PSI-NDH supercomplex in Hordeum vulgare.

Chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and plays an important role in mediating photosystem I (PSI) cyclic electron transport (CET)1-3. It associates with PSI to form the PSI-NDH supercomplex to fulfill its function. Here we report cryo-electron microscopy structure of a PSI-NDH supercomplex from barley (Hordeum vulgare) at an overall resolution of 4.4 Å and local resolutions of 3.40 Å-3.88 Å for the PSI-LHCI and NDH sub-complexes. The result reveals that PSI-NDH is composed of two copies of PSI-LHCI and one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast specific NDH subunits are observed and their exact positions as well as their interactions with other subunits in NDH are elucidated. Taken together, this study provides a structural basis for further investigations on the functions and regulation of the PSI-NDH-dependent CET.