Fischer N, Boudreau E, Hippler M, Drepper F, Haehnel W, Rochaix J D
Department of Molecular Biology, University of Geneva, Switzerland.
Biochemistry. 1999 Apr 27;38(17):5546-52. doi: 10.1021/bi982821a.
PsaJ is a small hydrophobic subunit of the photosystem I complex (PSI) whose function is not yet fully understood. Here we describe mutants of the green alga Chlamydomonas reinhardtii, in which the psaJ chloroplast gene has been inactivated either in a wild-type or in a PsaF-deficient nuclear background. Cells lacking one or both subunits grow photoautotrophically and contain normal levels of PSI. Flash-absorption spectroscopy performed with isolated PSI particles isolated from the PsaJ-deficient strain indicates that only 30% of the PSI complexes oxidize plastocyanin (Pc) or cytochrome c6 (Cyt c6) with kinetics identical to wild type, whereas the remaining 70% follow slow kinetics similar to those observed with PsaF-deficient PSI complexes. This feature is not due to partial loss of PsaF, as the PsaJ-less PSI complex contains normal levels of the PsaF subunit. The N-terminal domain of PsaF can be cross-linked to Pc and Cyt c6 indicating that in the absence of PsaJ, this domain is exposed in the lumenal space. Therefore, the decreased amount of functional PsaF revealed by the electron-transfer measurements is best explained by a displacement of the N-terminal domain of PsaF which is known to provide the docking site for Pc and Cyt c6. We propose that one function of PsaJ is to maintain PsaF in a proper orientation which allows fast electron transfer from soluble donor proteins to P700(+).
PsaJ是光系统I复合体（PSI）的一个小的疏水性亚基，其功能尚未完全明确。在此，我们描述了莱茵衣藻的突变体，其中psaJ叶绿体基因在野生型或PsaF缺陷型核背景中已被灭活。缺乏一个或两个亚基的细胞能够进行光合自养生长，并且含有正常水平的PSI。对从PsaJ缺陷型菌株中分离出的分离PSI颗粒进行的闪光吸收光谱分析表明，只有30%的PSI复合体以与野生型相同的动力学氧化质体蓝素（Pc）或细胞色素c6（Cyt c6），而其余70%则遵循与PsaF缺陷型PSI复合体中观察到的相似的慢动力学。这一特征并非由于PsaF的部分缺失，因为不含PsaJ的PSI复合体含有正常水平的PsaF亚基。PsaF的N端结构域可以与Pc和Cyt c6交联，这表明在没有PsaJ的情况下，该结构域暴露在腔空间中。因此，电子转移测量所揭示的功能性PsaF数量的减少，最好的解释是PsaF的N端结构域发生了位移，已知该结构域为Pc和Cyt c6提供对接位点。我们提出，PsaJ的一个功能是将PsaF维持在适当的方向，从而允许从可溶性供体蛋白到P700(+)的快速电子转移。
