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<p>Photosystem I is a key component of the solar energy conversion machinery in oxygenic photosynthesis, and its core, where photochemistry occurs, is highly conserved. However, the coral-associated alga <i>Chromera velia</i> that is evolutionary linked to parasitic apicomplexans, exhibits Photosystem I with unusual features. These include the splitting of the central PsaA subunit and the binding of superoxide dismutases as regular subunits. The organization of such a unique Photosystem I was enigmatic. Here, we present the cryo-EM structure of <i>Chromera velia</i> Photosystem I at 1.84 Å resolution. Our work reveals a superoxide dismutase heterodimer bound to the stromal side of the core, stabilized by extensions of canonical subunits, a novel protein PsaV, and a reduced light-harvesting apparatus. We elucidate how the complex evolved to accommodate the superoxide dismutase, assemble the split PsaA, and integrate antenna proteins in a non-canonical orientation. Based on our data and prior physiological data, we propose that this specialized Photosystem I functions likely as an Mehler machine, redirecting electrons from Photosystem II back to water. This mechanism enables <i>Chromera velia</i> to manage redox imbalance and reduce photorespiration through localized oxygen consumption.</p>

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The cryo-EM structure of Photosystem I from Chromera velia with a bound superoxide dismutase heterodimer

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年12月21日

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  • Xukun Yuan,
  • Martin Lukeš,
  • Pu Qian,
  • Quentin Charras,
  • Tomáš Smrčka,
  • Petr Man,
  • Lucie Kovářová,
  • Peter Koník,
  • Emmanuel Heilmann,
  • Ashraf Al-Amoudi,
  • Roman Sobotka,
  • Andreas Naschberger

摘要

Photosystem I is a key component of the solar energy conversion machinery in oxygenic photosynthesis, and its core, where photochemistry occurs, is highly conserved. However, the coral-associated alga Chromera velia that is evolutionary linked to parasitic apicomplexans, exhibits Photosystem I with unusual features. These include the splitting of the central PsaA subunit and the binding of superoxide dismutases as regular subunits. The organization of such a unique Photosystem I was enigmatic. Here, we present the cryo-EM structure of Chromera velia Photosystem I at 1.84 Å resolution. Our work reveals a superoxide dismutase heterodimer bound to the stromal side of the core, stabilized by extensions of canonical subunits, a novel protein PsaV, and a reduced light-harvesting apparatus. We elucidate how the complex evolved to accommodate the superoxide dismutase, assemble the split PsaA, and integrate antenna proteins in a non-canonical orientation. Based on our data and prior physiological data, we propose that this specialized Photosystem I functions likely as an Mehler machine, redirecting electrons from Photosystem II back to water. This mechanism enables Chromera velia to manage redox imbalance and reduce photorespiration through localized oxygen consumption.