Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel.
Photosythetic Antenna Research Center, Washington University in St. Louis, St. Louis, MO 63130, USA.
Biochim Biophys Acta Bioenerg. 2018 Jul;1859(7):544-553. doi: 10.1016/j.bbabio.2018.04.007. Epub 2018 Apr 25.
The major light harvesting antenna in all cyanobacterial species is the phycobilisome (PBS). The smallest PBS identified to date is that of Acaryochloris marina (A. marina), composed of a single four-hexamer rod. We have determined the crystal structure of phycocyanin (AmPC), the major component of the A. marina PBS (AmPBS) to 2.1 Å. The basic unit of the AmPC is a heterodimer of two related subunits (α and β), and we show that the asymmetric unit contains a superposition of two α and two β isoforms, the products of the simultaneous expression of different genes. This is the first time to our knowledge that isolated proteins crystallized with such identifiable heterogeneity. We believe that the presence of the different isoforms allows the AmPBS to have a significant bathochromic shift in its fluorescence emission spectrum, allowing, in the total absence of allophycocyanin, a better overlap with absorption of the chlorophyll d-containing reaction centers. We show that this bathochromic shift exists in intact AmPBS as well as in its disassembled components, thus suggesting that AmPC can efficiently serve as the AmPBS terminal emitter.
所有蓝细菌物种的主要光收集天线是藻胆体（PBS）。迄今为止鉴定出的最小 PBS 是海洋鱼腥藻（A. marina）的 PBS，由单个四聚体棒组成。我们已经确定了藻蓝蛋白（AmPC）的晶体结构，藻蓝蛋白是 A. marina PBS（AmPBS）的主要成分，分辨率为 2.1Å。AmPC 的基本单元是两个相关亚基（α和β）的异源二聚体，我们表明，不对称单元包含两个α和两个β同种型的叠加，这是同时表达不同基因的产物。据我们所知，这是首次在分离的蛋白质结晶中出现这种可识别的异质性。我们认为不同同种型的存在使得 AmPBS 在其荧光发射光谱中具有显著的红移，从而在完全没有别藻蓝蛋白的情况下，与含有叶绿素 d 的反应中心的吸收更好地重叠。我们表明，这种红移不仅存在于完整的 AmPBS 中，也存在于其拆开的组件中，因此表明 AmPC 可以有效地作为 AmPBS 的末端发射器。
