The Mina and Everard Goodman, Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel.
Plant J. 2018 Jun;94(5):813-821. doi: 10.1111/tpj.13896. Epub 2018 Apr 17.
Phycobilisomes, the macromolecular light harvesting complexes of cyanobacteria are degraded under nutrient-limiting conditions. This crucial response is required to adjust light excitation to the metabolic status and avoid damage by excess excitation. Phycobilisomes are comprised of phycobiliproteins, apo-proteins that covalently bind bilin chromophores. In the cyanobacterium Synechococcus elongatus, the phycobiliproteins allophycocyanin and phycocyanin comprise the core and the rods of the phycobilisome, respectively. Previously, NblB was identified as an essential component required for phycocyanin degradation under nutrient starvation. This protein is homologous to bilin-lyases, enzymes that catalyze the covalent attachment of bilins to apo-proteins. However, the nblB-inactivated strain is not impaired in phycobiliprotein synthesis, but rather is characterized by aberrant phycocyanin degradation. Here, using a phycocyanin-deficient strain, we demonstrate that NblB is required for degradation of the core pigment, allophycocyanin. Furthermore, we show that the protein NblB is expressed under nutrient sufficient conditions, but during nitrogen starvation its level decreases about two-fold. This finding is in contrast to an additional component essential for degradation, NblA, the expression of which is highly induced under starvation. We further identified NblB residues required for phycocyanin degradation in vivo. Finally, we demonstrate phycocyanin degradation in a cell-free system, thereby providing support for the suggestion that NblB directly mediates pigment degradation by chromophore detachment. The dependence of NblB function on NblA revealed using this system, together with the results indicating presence of NblB under nutrient sufficient conditions, suggests a rapid mechanism for induction of pigment degradation, which requires only the expression of NblA.
藻胆体是蓝藻的高分子光捕获复合物，在营养限制条件下会降解。这种关键的反应是必需的，以调整光激发与代谢状态，并避免因过度激发而造成的损伤。藻胆体由藻胆蛋白组成，是与发色团共价结合的脱辅基蛋白。在蓝藻聚球藻中，藻胆蛋白藻蓝蛋白和藻红蛋白分别构成藻胆体的核心和棒。先前，NblB 被鉴定为在营养饥饿下藻红蛋白降解所必需的重要组成部分。这种蛋白质与胆红素裂合酶同源，后者是催化胆红素与脱辅基蛋白共价结合的酶。然而，nblB 失活的菌株在藻胆蛋白合成中没有受损，而是表现出藻红蛋白降解异常。在这里，我们使用藻蓝蛋白缺陷株证明了 NblB 是核心色素藻蓝蛋白降解所必需的。此外，我们表明，在营养充足的条件下表达蛋白 NblB，但在氮饥饿时其水平降低约两倍。这一发现与另一个降解所必需的成分 NblA 形成对比，NblA 的表达在饥饿时高度诱导。我们进一步鉴定了 NblB 在体内降解藻红蛋白所必需的残基。最后，我们在无细胞系统中证明了藻红蛋白的降解，从而为 NblB 通过发色团脱离直接介导色素降解的观点提供了支持。使用该系统揭示了 NblB 功能对 NblA 的依赖性，以及在营养充足条件下存在 NblB 的结果表明，诱导色素降解的机制迅速，只需要 NblA 的表达。
