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<p>Cryptophyte algae are an important group of photosynthetic eukaryotes and originated evolutionarily from a secondary endosymbiosis with a red algal ancestor. Photosystem I (PSI) of cryptophyte algae binds alloxanthin-chlorophyll <i>a</i>/<i>c</i>-binding proteins (ACPs) equivalent to the light harvesting complexes (LHCs) of green algae and higher plants. Here, we report a high resolution (2.25 Å) structure of the PSI-ACPI supercomplex purified from the cryptophyte <i>Chroomonas placoidea</i> (<i>C. placoidea</i>) by cryo-electron microscopy. The PSI-ACPI supercomplex consists of a monomeric PSI core, 14 ACPI subunits and a Chl-binding subunit, CCPI-S. The 14 ACPI subunits are divided into two layers, with an inner layer including 11 ACPIs, and an outer layer consisting of 3 ACPIs. The CCPI-S mediates the connections and energy transfer between the outer and inner ACPI layers. Due to the current higher resolution, a unique PsaQ subunit bound at the lumenal side was identified in the PSI core. Two new pigment molecules Chl <i>a</i>856<sub>PsaA</sub> and Mon 627<sub>ACPI − 8</sub> as well as 18 new lipids were identified, compared to the previously reported PSI-ACPI structure from the cryptophyte alga <i>C. placoidea</i> (Zhao et al. <CitationRef CitationID="CR63">2023</CitationRef>). These lipids mediate the interactions of subunits and the assembly of PSI-ACPI. These results provide structural insights for the assembly of the PSI-ACPI complex and pathways of light-energy absorption and transfer in cryptophyte PSI-ACPI, as well as the possible adaptation mechanisms to fluctuating light conditions.</p>

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Structural insights into a PSI-ACPI supercomplex from a cryptophyte alga Chroomonas placoidea

  • 期刊论文

  • 发表日期：

    2025年12月20日

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  • Xingyue Li,
  • Zhiyuan Mao,
  • Zhenhua Li,
  • Liangliang Shen,
  • Xiaoyi Li,
  • Yanyan Yang,
  • Wenda Wang,
  • Tingyun Kuang,
  • Jian-Ren Shen,
  • Guangye Han

摘要

Cryptophyte algae are an important group of photosynthetic eukaryotes and originated evolutionarily from a secondary endosymbiosis with a red algal ancestor. Photosystem I (PSI) of cryptophyte algae binds alloxanthin-chlorophyll a/c-binding proteins (ACPs) equivalent to the light harvesting complexes (LHCs) of green algae and higher plants. Here, we report a high resolution (2.25 Å) structure of the PSI-ACPI supercomplex purified from the cryptophyte Chroomonas placoidea (C. placoidea) by cryo-electron microscopy. The PSI-ACPI supercomplex consists of a monomeric PSI core, 14 ACPI subunits and a Chl-binding subunit, CCPI-S. The 14 ACPI subunits are divided into two layers, with an inner layer including 11 ACPIs, and an outer layer consisting of 3 ACPIs. The CCPI-S mediates the connections and energy transfer between the outer and inner ACPI layers. Due to the current higher resolution, a unique PsaQ subunit bound at the lumenal side was identified in the PSI core. Two new pigment molecules Chl a856PsaA and Mon 627ACPI − 8 as well as 18 new lipids were identified, compared to the previously reported PSI-ACPI structure from the cryptophyte alga C. placoidea (Zhao et al. 2023). These lipids mediate the interactions of subunits and the assembly of PSI-ACPI. These results provide structural insights for the assembly of the PSI-ACPI complex and pathways of light-energy absorption and transfer in cryptophyte PSI-ACPI, as well as the possible adaptation mechanisms to fluctuating light conditions.