Pagliano Cristina, La Rocca Nicoletta, Andreucci Flora, Deák Zsuzsanna, Vass Imre, Rascio Nicoletta, Barbato Roberto
Dipartimento di Scienze dell'Ambiente e della Vita, Università del Piemonte Orientale, Alessandria, Italy.
Ann Bot. 2009 Feb;103(3):505-15. doi: 10.1093/aob/mcn234. Epub 2008 Nov 25.
Photosystem II of oxygenic organisms is a multi-subunit protein complex made up of at least 20 subunits and requires Ca(2+) and Cl(-) as essential co-factors. While most subunits form the catalytic core responsible for water oxidation, PsbO, PsbP and PsbQ form an extrinsic domain exposed to the luminal side of the membrane. In vitro studies have shown that these subunits have a role in modulating the function of Cl(-) and Ca(2+), but their role(s) in vivo remains to be elucidated, as the relationships between ion concentrations and extrinsic polypeptides are not clear. With the aim of understanding these relationships, the photosynthetic apparatus of the extreme halophyte Salicornia veneta has been compared with that of spinach. Compared to glycophytes, halophytes have a different ionic composition, which could be expected to modulate the role of extrinsic polypeptides.
Structure and function of in vivo and in vitro PSII in S. veneta were investigated and compared to spinach. Light and electron microscopy, oxygen evolution, gel electrophoresis, immunoblotting, DNA sequencing, RT-PCR and time-resolved chlorophyll fluorescence were used.
Thylakoids of S. veneta did not contain PsbQ protein and its mRNA was absent. When compared to spinach, PsbP was partly depleted (30 %), as was its mRNA. All other thylakoid subunits were present in similar amounts in both species. PSII electron transfer was not affected. Fluorescence was strongly quenched upon irradiation of plants with high light, and relaxed only after prolonged dark incubation. Quenching of fluorescence was not linked to degradation of D1 protein.
In S. veneta the PsbQ protein is not necessary for photosynthesis in vivo. As the amount of PsbP is sub-stoichiometric with other PSII subunits, this protein too is largely dispensable from a catalytic standpoint. One possibility is that PsbP acts as an assembly factor for PSII.
产氧生物的光系统II是一种由至少20个亚基组成的多亚基蛋白复合体，需要Ca(2+)和Cl(-)作为必需的辅助因子。虽然大多数亚基形成负责水氧化的催化核心，但PsbO、PsbP和PsbQ形成暴露于膜腔侧的外周结构域。体外研究表明，这些亚基在调节Cl(-)和Ca(2+)的功能中起作用，但它们在体内的作用仍有待阐明，因为离子浓度与外周多肽之间的关系尚不清楚。为了理解这些关系，将极端盐生植物盐角草的光合装置与菠菜的进行了比较。与甜土植物相比，盐生植物具有不同的离子组成，这可能会调节外周多肽的作用。
研究了盐角草体内和体外PSII的结构与功能，并与菠菜进行了比较。使用了光镜和电镜、放氧、凝胶电泳、免疫印迹、DNA测序、RT-PCR和时间分辨叶绿素荧光技术。
盐角草的类囊体不含PsbQ蛋白且其mRNA缺失。与菠菜相比，PsbP部分缺失（30%），其mRNA也是如此。两种植物中所有其他类囊体亚基的含量相似。PSII电子传递未受影响。高光照射植物时荧光强烈猝灭，仅在长时间暗培养后才恢复。荧光猝灭与D1蛋白的降解无关。
在盐角草中，PsbQ蛋白在体内光合作用中并非必需。由于PsbP的量与其他PSII亚基的化学计量比不足，从催化角度来看，这种蛋白在很大程度上也是可有可无的。一种可能性是PsbP作为PSII的组装因子发挥作用。
