Department of Biology, Washington University, St. Louis, MO 63130, USA.
Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA.
Biomolecules. 2019 Aug 16;9(8):374. doi: 10.3390/biom9080374.
Phycobilisomes (PBSs) are large (3-5 megadalton) pigment-protein complexes in cyanobacteria that associate with thylakoid membranes and harvest light primarily for photosystem II. PBSs consist of highly ordered assemblies of pigmented phycobiliproteins (PBPs) and linker proteins that can account for up to half of the soluble protein in cells. Cyanobacteria adjust to changing environmental conditions by modulating PBS size and number. In response to nutrient depletion such as nitrogen (N) deprivation, PBSs are degraded in an extensive, tightly controlled, and reversible process. In UTEX 2973, a fast-growing cyanobacterium with a doubling time of two hours, the process of PBS degradation is very rapid, with 80% of PBSs per cell degraded in six hours under optimal light and CO conditions. Proteomic analysis during PBS degradation and re-synthesis revealed multiple proteoforms of PBPs with partially degraded phycocyanobilin (PCB) pigments. NblA, a small proteolysis adaptor essential for PBS degradation, was characterized and validated with targeted mass spectrometry. NblA levels rose from essentially 0 to 25,000 copies per cell within 30 min of N depletion, and correlated with the rate of decrease in phycocyanin (PC). Implications of this correlation on the overall mechanism of PBS degradation during N deprivation are discussed.
藻胆体（PBS）是蓝细菌中一种大型（3-5 兆道尔顿）的色素-蛋白复合物，与类囊体膜结合，主要用于光系统 II 收集光。PBS 由高度有序排列的色素藻胆蛋白（PBPs）和连接蛋白组成，其含量可高达细胞可溶性蛋白的一半。蓝细菌通过调节 PBS 的大小和数量来适应不断变化的环境条件。在响应营养物质（如氮）耗尽等情况时，PBS 会经历广泛、严格控制且可逆的降解过程。在生长迅速的蓝细菌 UTEX 2973 中，倍增时间为两小时，PBS 的降解过程非常迅速，在最佳光照和 CO 条件下，细胞内 80%的 PBS 在六小时内降解。在 PBS 降解和重新合成过程中的蛋白质组学分析揭示了 PBPs 的多种蛋白异构体，其藻胆素（PCB）色素部分降解。NblA 是 PBS 降解所必需的小蛋白水解衔接蛋白，通过靶向质谱法对其进行了鉴定和验证。在 N 耗尽的 30 分钟内，NblA 水平从基本为 0 上升到每个细胞 25,000 个拷贝，与藻蓝蛋白（PC）的减少速率相关。讨论了这种相关性对 N 缺乏期间 PBS 降解总体机制的影响。
