Mozzo Milena, Morosinotto Tomas, Bassi Roberto, Croce Roberta
Istituto di Biofisica. CNR. C/o ITC via Sommarvie 18. 38100 Povo. Trento, Italy.
Biochim Biophys Acta. 2006 Dec;1757(12):1607-13. doi: 10.1016/j.bbabio.2006.06.018. Epub 2006 Jul 21.
Lhc proteins constitute a family of transmembrane proteins which share homology in sequence and similarity in the general organisation although members can be strongly differentiated such as in the case of PsbS and ELIPs. In this work, we report on the structure of Lhca3, a pigment-protein subunit component of the antenna system of higher plants Photosystem I, through the effect of point mutations in critical sites. Based on the structure of PSI-LHCI (Ben Shem et al., PDB file 1QZV remark 999) it has been suggested that Lhca3 may have different folding as compared to other members of the Lhc family. In particular, it was proposed that the two central helices may be swapped and chlorophylls in sites 1013 and 1023 are not present. This different folding would imply that the chlorophylls coordinated to the two central helices have different ligands in Lhca3 with respect to the other Lhc complexes. The structural model was tested by substituting the putative binding residues with residues unable to coordinate chlorophyll and the spectroscopic properties of the individual pigments were used as structural probes. The results indicate that Lhca3 folds in the same way as the other antenna proteins. Moreover, the low-energy absorption form originates from interaction between chlorophylls in site 1015 and 1025, like for the other PSI antenna subunits. Evidence is also shown for the presence in Lhca3 of chlorophylls in sites 1013 and 1023.
Lhc蛋白构成了一个跨膜蛋白家族，尽管成员之间可能存在很大差异，如PsbS和ELIPs的情况，但它们在序列上具有同源性，在总体结构上具有相似性。在这项工作中，我们通过关键位点的点突变效应，报道了高等植物光系统I天线系统的色素蛋白亚基组分Lhca3的结构。基于PSI-LHCI的结构（本·舍姆等人，PDB文件1QZV备注999），有人提出Lhca3与Lhc家族的其他成员相比可能具有不同的折叠方式。特别是，有人提出两个中心螺旋可能会互换，并且位点1013和1023处不存在叶绿素。这种不同的折叠方式意味着与两个中心螺旋配位的叶绿素在Lhca3中相对于其他Lhc复合物具有不同的配体。通过用无法配位叶绿素的残基取代假定的结合残基来测试结构模型，并将各个色素的光谱性质用作结构探针。结果表明，Lhca3的折叠方式与其他天线蛋白相同。此外，低能吸收形式源于位点1015和1025处叶绿素之间的相互作用，就像其他PSI天线亚基一样。也有证据表明Lhca3在位点1013和1023处存在叶绿素。
