Xu Qian-Zhao, Han Jia-Xin, Tang Qi-Ying, Ding Wen-Long, Miao Dan, Zhou Ming, Scheer Hugo, Zhao Kai-Hong
State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, PR China.
Department Biologie I, Universität München, Menzinger Str. 67, D-80638 München, Germany.
Biochim Biophys Acta. 2016 Sep;1857(9):1607-1616. doi: 10.1016/j.bbabio.2016.06.008. Epub 2016 Jun 29.
Cyanobacterial light-harvesting complexes, phycobilisomes, can undergo extensive remodeling under varying light conditions. Acclimation to far-red light involves not only generation of red-shifted chlorophylls in the photosystems, but also induction of additional copies of core biliproteins that have been related to red-shifted components of the phycobilisome (Gan et al., Life 5, 4, 2015). We are studying the molecular basis for these acclimations in Chroococcidiopsis thermalis sp. PCC7203. Five far-red induced allophycocyanin subunits (ApcA2, ApcA3, ApcB2, ApcB3 and ApcF2) were expressed in Escherichia coli, together with S-type chromophore-protein lyases and in situ generated chromophore, phycocyanobilin. Only one subunit, ApcF2, shows an unusual red-shift (λAmax675nm, λFmax698nm): it binds the chromophore non-covalently, thereby preserving its full conjugation length. This mechanism operates also in two Cys-variants of the induced subunits of bulky APC. All other wild-type subunits bind phycocyanobilin covalently to the conventional Cys-81 under catalysis of the lyase, CpcS1. Although three of them also show binding to additional cysteines, all absorb and fluoresce similar to conventional APC subunits (λAmax610nm, λFmax640nm). Another origin of red-shifted complexes was identified, however, when different wild-type α- and β-subunits of the far-red induced bulky APC were combined in a combinatorial fashion. Strongly red-shifted complexes (λFmax≤722nm) were formed when the α-subunit, PCB-ApcA2, and the β-subunit, PCB-ApcB2, were generated together in E. coli. This extreme aggregation-induced red-shift of ~90nm of covalently bound chromophores is reminiscent, but much larger, than the ~30nm observed with conventional APC.
蓝藻光捕获复合体——藻胆体，可在不同光照条件下进行广泛重塑。适应远红光不仅涉及光系统中红移叶绿素的产生，还包括诱导与藻胆体红移成分相关的核心胆色素蛋白额外拷贝的生成（Gan等人，《生命》5卷，4期，2015年）。我们正在研究嗜热栖热菌（Chroococcidiopsis thermalis）sp. PCC7203中这些适应性变化的分子基础。五个远红光诱导的别藻蓝蛋白亚基（ApcA2、ApcA3、ApcB2、ApcB3和ApcF2）与S型发色团-蛋白裂合酶以及原位生成的发色团藻蓝胆素一起在大肠杆菌中表达。只有一个亚基ApcF2呈现出异常的红移（λAmax675nm，λFmax698nm）：它以非共价方式结合发色团，从而保留其完整的共轭长度。这种机制在大体积别藻蓝蛋白诱导亚基的两个半胱氨酸变体中也起作用。所有其他野生型亚基在裂合酶CpcS1的催化下，将藻蓝胆素共价结合到常规的半胱氨酸-81上。尽管其中三个亚基也显示出与其他半胱氨酸的结合，但它们的吸收和荧光都与常规别藻蓝蛋白亚基相似（λAmax610nm，λFmax640nm）。然而，当以组合方式将远红光诱导的大体积别藻蓝蛋白的不同野生型α和β亚基组合时，发现了红移复合体的另一个来源。当α亚基PCB-ApcA2和β亚基PCB-ApcB2在大肠杆菌中一起产生时，形成了强烈红移的复合体（λFmax≤722nm）。这种共价结合发色团约90nm的极端聚集诱导红移，与常规别藻蓝蛋白观察到的约30nm红移相似，但程度要大得多。
