Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA.
Biochemistry. 2010 Mar 2;49(8):1744-54. doi: 10.1021/bi901591m.
Ppr is a unique bacteriophytochrome that bleaches rather than forming a far-red-shifted Pfr state upon red light activation. Ppr is also unusual in that it has a blue light photoreceptor domain, PYP, which is N-terminally fused to the bacteriophytochrome domain (Bph). When both photoreceptors are activated by light, the fast phase of Bph recovery (1 min lifetime) corresponds to the formation of an intramolecular long-lived complex between the activated PYP domain and the Bph domain (lifetime of 2-3 days). Since this state is unusually long-lived as compared to other intermediates in the photocycle of both PYP and Bph, we interpret this as formation of a metastable complex between activated PYP and Bph domains that takes days to relax. In the metastable complex, the PYP domain is locked in its activated UV absorbing state and the Bph domain is in a slightly red-shifted state (from 701 to 702 nm), which is photochemically inactive to red or white light. The amount of metastable complex formed increases with the degree of prior activation of PYP, reaching a maximum of 50% when PYP is fully activated compared to 0% when no PYP is activated. The saturation of complex formation at 50% is believed to be due to light-induced heterogeneity within the Ppr dimer. UV irradiation (365 nm) of the metastable complex state photoreverses the activated PYP and the red-shifted Bph to the initial dark state within seconds. We therefore postulate that Ppr functions as a UV-red light sensor and describe the different Ppr states that can be obtained depending on the light quality. Both red and white light upregulate the autokinase activity, while it is downregulated in the dark. The physiological state of Ppr is most likely a mixture of three different states, dark, metastable complex, and red light-activated, with fractional populations whose amounts depend on the light quality of the environment and that regulate the extent of phosphorylation by the kinase.
ppr 是一种独特的细菌视紫红质，在红光激活时会漂白，而不是形成远红移的 Pfr 状态。ppr 也很不寻常，因为它有一个蓝光光受体结构域 PYP，它与细菌视紫红质结构域（Bph）的 N 端融合。当两个光受体都被光激活时，Bph 恢复的快速相（1 分钟寿命）对应于激活的 PYP 结构域和 Bph 结构域之间形成长寿命的分子内复合物（寿命为 2-3 天）。由于与 PYP 和 Bph 光循环中的其他中间体相比，这种状态的寿命异常长，我们将其解释为激活的 PYP 和 Bph 结构域之间形成了一个亚稳态复合物，需要数天才能松弛。在亚稳态复合物中，PYP 结构域被锁定在其激活的紫外吸收状态，Bph 结构域处于略微红移状态（从 701nm 到 702nm），对红光或白光没有光化学活性。形成的亚稳态复合物的量随 PYP 先前激活程度的增加而增加，当 PYP 完全激活时达到最大值 50%，而当没有 PYP 激活时达到 0%。复合物形成的饱和达到 50%，这被认为是由于 Ppr 二聚体中的光诱导异质性。亚稳态复合物状态的 UV 辐射（365nm）可在数秒内将激活的 PYP 和红移的 Bph 光逆转回初始暗态。因此，我们假设 ppr 作为 UV-红光传感器发挥作用，并描述根据光质可以获得的不同 ppr 状态。红光和白光都上调了自激酶活性，而在黑暗中则下调。ppr 的生理状态很可能是三种不同状态的混合物，即黑暗、亚稳态复合物和红光激活，其分数种群数量取决于环境的光质，并调节激酶的磷酸化程度。
