Maor-Landaw Keren, Eisenhut Marion, Tortorelli Giada, van de Meene Allison, Kurz Samantha, Segal Gabriela, van Oppen Madeleine J H, Weber Andreas P M, McFadden Geoffrey I
School of Biosciences, The University of Melbourne, Parkville, VIC, Australia.
Department of Marine Biology, The Leon H. Charney School of Marine Sciences, University of Haifa, Haifa, Israel.
ISME Commun. 2023 Jan 28;3(1):7. doi: 10.1038/s43705-023-00218-8.
The symbiotic partnership between corals and dinoflagellate algae is crucial to coral reefs. Corals provide their algal symbionts with shelter, carbon dioxide and nitrogen. In exchange, the symbiotic algae supply their animal hosts with fixed carbon in the form of glucose. But how glucose is transferred from the algal symbiont to the animal host is unknown. We reasoned that a transporter resident in the dinoflagellate cell membrane would facilitate outward transfer of glucose to the surrounding host animal tissue. We identified a candidate transporter in the cnidarian symbiont dinoflagellate Breviolum minutum that belongs to the ubiquitous family of facilitative sugar uniporters known as SWEETs (sugars will eventually be exported transporters). Previous gene expression analyses had shown that BmSWEET1 is upregulated when the algae are living symbiotically in a cnidarian host by comparison to the free-living state [1, 2]. We used immunofluorescence microscopy to localise BmSWEET1 in the dinoflagellate cell membrane. Substrate preference assays in a yeast surrogate transport system showed that BmSWEET1 transports glucose. Quantitative microscopy showed that symbiotic B. minutum cells have significantly more BmSWEET1 protein than free-living cells of the same strain, consistent with export during symbiosis but not during the free-living, planktonic phase. Thus, BmSWEET1 is in the right place, at the right time, and has the right substrate to be the transporter with which symbiotic dinoflagellate algae feed their animal hosts to power coral reefs.
珊瑚与甲藻之间的共生关系对珊瑚礁至关重要。珊瑚为其藻类共生体提供庇护所、二氧化碳和氮。作为交换，共生藻类以葡萄糖的形式为其动物宿主提供固定碳。但葡萄糖是如何从藻类共生体转移到动物宿主的尚不清楚。我们推测，存在于甲藻细胞膜中的一种转运蛋白会促进葡萄糖向外转移至周围的宿主动物组织。我们在刺胞动物共生体微小布氏藻中鉴定出一种候选转运蛋白，它属于普遍存在的易化性单糖转运蛋白家族，即SWEETs（糖最终输出转运蛋白）。先前的基因表达分析表明，与自由生活状态相比，当藻类在刺胞动物宿主中共生生活时，BmSWEET1的表达会上调[1,2]。我们利用免疫荧光显微镜将BmSWEET1定位在甲藻细胞膜中。在酵母替代转运系统中进行的底物偏好测定表明，BmSWEET1转运葡萄糖。定量显微镜分析显示共生的微小布氏藻细胞比同一菌株的自由生活细胞具有显著更多的BmSWEET1蛋白，这与共生期间而非自由生活的浮游阶段的输出情况一致。因此，BmSWEET1在正确的位置、正确的时间，并且具有正确的底物，有望成为共生甲藻为其动物宿主提供能量以维持珊瑚礁的转运蛋白。
