Institute for Protein Research, Osaka University, Suita, Osaka 565–0871, Japan; Institute for Open and Transdisciplinary Research Initiatives (OTRI), Osaka University, Suita, Osaka 565–9871, Japan
Hideaki Tanaka
Institute for Protein Research, Osaka University, Suita, Osaka 565–0871, Japan; Institute for Open and Transdisciplinary Research Initiatives (OTRI), Osaka University, Suita, Osaka 565–9871, Japan
Institute for Protein Research, Osaka University, Suita, Osaka 565–0871, Japan; Institute for Open and Transdisciplinary Research Initiatives (OTRI), Osaka University, Suita, Osaka 565–9871, Japan
Ritsuko Fujii
Division of Molecular Materials Science, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558–8585, Japan; Department of Chemistry, Graduate School of Science, Osaka Metropolitan University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558–8585, Japan; Research Center for Artificial Photosynthesis (ReCAP), Osaka Metropolitan University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558–8585, Japan; Corresponding author.
Light-harvesting complex II (LHCII) present in plants and green algae absorbs solar energy to promote photochemical reactions. A marine green macroalga, Codium fragile, exhibits the unique characteristic of absorbing blue-green light from the sun during photochemical reactions while being underwater owing to the presence of pigment-altered LHCII called siphonaxanthin–chlorophyll a/b-binding protein (SCP). In this study, we determined the structure of SCP at a resolution of 2.78 Å using cryogenic electron microscopy. SCP has a trimeric structure, wherein each monomer containing two lutein and two chlorophyll a molecules in the plant-type LHCII are replaced by siphonaxanthin and its ester and two chlorophyll b molecules, respectively. Siphonaxanthin occupies the binding site in SCP having a polarity in the trimeric inner core, and exhibits a distorted conjugated chain comprising a carbonyl group hydrogen bonded to a cysteine residue of apoprotein. These features suggest that the siphonaxanthin molecule is responsible for the characteristic green absorption of SCP. The replaced chlorophyll b molecules extend the region of the stromal side chlorophyll b cluster, spanning two adjacent monomers.
