Chen Min, Ma Wutunan, Michelle Tiarne
School of Life and Environmental Sciences, The University of Sydney, Sydney, Australia.
Photosynth Res. 2025 Jul 21;163(4):40. doi: 10.1007/s11120-025-01160-7.
Allophycocyanin (APC) protein subunits responsible for red-shifted phycobilisomes are induced under far-red light conditions. The ApcB2 (encoded by gene XM38_020890) and ApcD4 (encoded by gene XM38_020900) in H. hongdechloris are paralogous APC subunits encoded in the termed low-light photoacclimation (LoLiP) gene cluster, which are only detected from cells grown under far-red light (FRL) conditions. We examined the function of these allophycocyanin subunits using heterogeneous recombinant E. coli systems. The recombinant chromophorylated ApcB2 showed absorptionpeaking at 618 nm and fluorescence peaking at 642 nm and the chromophorylated ApcD4 demonstrated two absorption peaks of 618 and 676 nm and fluorescence peaks of 625 and 698 nm, respectively. Interestingly, the heterodimer of ApcB2/ApcD4 demonstrated even further FRL absorption of 728 nm and fluorescence emission peaking at 742 nm. Using ΔApcB2-W75T to replace ApcB2 for APC ab heterodimeric formation, the red-shifted absorption at 728 nm disappeared, suggesting that Trp75 of ApcB2 is essential for the heterodimer maintaining the red-shifted 728 nm spectroscopic feature. The extremely red-shifted spectroscopic properties of ApcD4/ApcB2 complexes reveal the strain-specific diversity of FRL-phycobilisomes and advance our understanding of remodelled light-harvesting complexes that capture FRL. In H. hongdechloris, besides the well-known Far-red light Photoacclimation (FaRLiP) gene cluster, the APC αβ heterodimer of ApcB2/ApcD4 from LoLiP gene cluster likely functions as the terminal emitter of red-shifted phycobilisomes for chlorophyll f-binding protein complexes. The recombinant, red-shifted APC αβ heterodimer offers a potential new class of fluorescence labels in the near-infrared spectral region.
负责红移藻胆体的别藻蓝蛋白（APC）蛋白亚基在远红光条件下被诱导。洪氏嗜盐绿菌中的ApcB2（由基因XM38_020890编码）和ApcD4（由基因XM38_020900编码）是编码在所谓的低光光适应（LoLiP）基因簇中的旁系同源APC亚基，仅在远红光（FRL）条件下生长的细胞中检测到。我们使用异源重组大肠杆菌系统研究了这些别藻蓝蛋白亚基的功能。重组的发色磷酸化ApcB2在618nm处有吸收峰，在642nm处有荧光峰，发色磷酸化的ApcD4分别有618和676nm的两个吸收峰以及625和698nm的荧光峰。有趣的是，ApcB2/ApcD4异二聚体表现出甚至更远红光吸收峰728nm以及荧光发射峰742nm。使用ΔApcB2-W75T替代ApcB2形成APC ab异二聚体时，728nm处的红移吸收消失，这表明ApcB2的Trp75对于异二聚体维持728nm红移光谱特征至关重要。ApcD4/ApcB2复合物的极红移光谱特性揭示了FRL-藻胆体的菌株特异性多样性，并推进了我们对捕获FRL的重塑光捕获复合物的理解。在洪氏嗜盐绿菌中，除了众所周知的远红光光适应（FaRLiP）基因簇外，来自LoLiP基因簇的ApcB2/ApcD4的APCαβ异二聚体可能作为用于叶绿素f结合蛋白复合物的红移藻胆体的末端发射体。重组的、红移的APCαβ异二聚体在近红外光谱区域提供了一类潜在的新型荧光标记。
