Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K.
Biochem J. 2018 Apr 5;475(7):1225-1233. doi: 10.1042/BCJ20170526.
The photosynthetic chloroplast thylakoid membrane of higher plants is a complex three-dimensional structure that is morphologically dynamic on a timescale of just a few minutes. The membrane dynamics are driven by the phosphorylation of light-harvesting complex II (LHCII) by the STN7 kinase, which controls the size of the stacked grana region relative to the unstacked stromal lamellae region. Here, I hypothesise that the functional significance of these membrane dynamics is in controlling the partition of electrons between photosynthetic linear and cyclic electron transfer (LET and CET), which determines the ratio of NADPH/ATP produced. The STN7 kinase responds to the metabolic state of the chloroplast by sensing the stromal redox state. A high NADPH/ATP ratio leads to reduction of thioredoxin f (TRX), which reduces a CxxxC motif in the stromal domain of STN7 leading to its inactivation, whereas a low NADPH/ATP ratio leads to oxidation of TRX and STN7 activation. Phosphorylation of LHCII leads to smaller grana, which favour LET by speeding up diffusion of electron carriers plastoquinone (PQ) and plastocyanin (PC) between the domains. In contrast, dephosphorylation of LHCII leads to larger grana that slow the diffusion of PQ and PC, leaving the PQ pool in the stroma more oxidised, thus enhancing the efficiency of CET. The feedback regulation of electron transfer by the downstream metabolism is crucial to plant fitness, since perturbations in the NADPH/ATP ratio can rapidly lead to the inhibition of photosynthesis and photo-oxidative stress.
高等植物的光合叶绿体类囊体膜是一种复杂的三维结构，其形态在短短几分钟的时间尺度上就具有动态变化。膜的动态变化是由 STN7 激酶磷酸化光捕获复合物 II（LHCII）驱动的，该激酶控制堆叠的粒状区域相对于未堆叠的基质层区域的大小。在这里，我假设这些膜动态变化的功能意义在于控制电子在光合线性和循环电子传递（LET 和 CET）之间的分配，这决定了 NADPH/ATP 的产生比例。STN7 激酶通过感知基质的氧化还原状态来响应叶绿体的代谢状态。高 NADPH/ATP 比例导致硫氧还蛋白 f（TRX）还原，从而减少 STN7 基质结构域中的 CxxxC 基序，导致其失活，而低 NADPH/ATP 比例导致 TRX 氧化和 STN7 激活。LHCII 的磷酸化导致较小的粒状结构，通过加速电子载体质体醌（PQ）和质体蓝素（PC）在域之间的扩散，有利于 LET。相比之下，LHCII 的去磷酸化导致较大的粒状结构，减缓 PQ 和 PC 的扩散，使基质中的 PQ 池更氧化，从而提高 CET 的效率。电子传递的下游代谢反馈调节对植物的适应性至关重要，因为 NADPH/ATP 比例的波动会迅速导致光合作用和光氧化应激的抑制。
