Structural insights into the unusual core photocomplex from a triply extremophilic purple bacterium,Halorhodospira halochloris

Halorhodospira(Hlr.)halochloris is a triply extremophilic phototrophic purple sulfur bacterium,as it is thermophilic,alkaliphilic,and extremely halophilic.The light-harvesting-reaction center(LH1–RC)core complex of this bacterium displays an LH1-Q_(y)transition at 1,016 nm,which is the lowest-energy wavelength absorption among all known phototrophs.Here we report the cryo-EM structure of the LH1–RC at 2.42?resolution.The LH1 complex forms a tricyclic ring structure composed of 16αβγ-polypeptides and oneαβ-heterodimer around the RC.From the cryo-EM density map,two previously unrecognized integral membrane proteins,referred to as protein G and protein Q,were identified.Both of these proteins are single transmembrane-spanning helices located between the LH1 ring and the RC Lsubunit and are absent from the LH1–RC complexes of all other purple bacteria of which the structures have been determined so far.Besides bacteriochlorophyll b molecules(B1020)located on the periplasmic side of the Hlr.halochloris membrane,there are also two arrays of bacteriochlorophyll b molecules(B800 and B820)located on the cytoplasmic side.Only a single copy of a carotenoid(lycopene)was resolved in the Hlr.halochloris LH1–α3β3 and this was positioned within the complex.The potential quinone channel should be the space between the LH1–α3β3 that accommodates the single lycopene but does not contain aγ-polypeptide,B800 and B820.Our results provide a structural explanation for the unusual Q_(y)red shift and carotenoid absorption in the Hlr.halochloris spectrum and reveal new insights into photosynthetic mechanisms employed by a species that thrives under the harshest conditions of any phototrophic microorganism known.

Structure of plant photosystem I−light harvesting complex I supercomplex at 2.4Å resolution

Photosystem I(PSI)is one of the two photosystems in photosynthesis,and performs a series of electron transfer reactions leading to the reduction of ferredoxin.In higher plants,PSI is surrounded by four light-harvesting complex I(LHCI)subunits,which harvest and transfer energy efficiently to the PSI core.The crystal structure of PSI-LHCI supercomplex has been analyzed up to 2.6Åresolution,providing much information on the arrangement of proteins and cofactors in this complicated supercomplex.Here we have optimized crystallization conditions,and analyzed the crystal structure of PSI-LHCI at 2.4Åresolution.Our structure showed some shift of the LHCI,especially the Lhca4 subunit,away from the PSI core,suggesting the indirect connection and inefficiency of energy transfer from this Lhca subunit to the PSI core.We identified five new lipids in the structure,most of them are located in the gap region between the Lhca subunits and the PSI core.These lipid molecules may play important roles in binding of the Lhca subunits to the core,as well as in the assembly of the supercomplex.The present results thus provide novel information for the elucidation of the mechanisms for the light-energy harvesting,transfer and assembly of this supercomplex.