The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, The University of Edinburgh, Daniel Rutherford Building, The King's Buildings, Max Born Crescent, Edinburgh EH9 3BF, UK.
BASF Innovation Center Gent- Trait Research, Technologiepark-Zwijnaarde, 9052 Gent, Belgium.
J Plant Physiol. 2020 Aug;251:153210. doi: 10.1016/j.jplph.2020.153210. Epub 2020 Jun 4.
Transglycanases are enzymes that remodel the primary cell wall in plants, potentially loosening and/or strengthening it. Xyloglucan endotransglucosylase (XET; EC 2.4.1.207), ubiquitous in land plants, is a homo-transglucanase activity (donor, xyloglucan; acceptor, xyloglucan) exhibited by XTH (xyloglucan endotransglucosylase/hydrolase) proteins. By contrast, hetero-trans-β-glucanase (HTG) is the only known enzyme that is preferentially a hetero-transglucanase. Its two main hetero-transglucanase activities are MLG : xyloglucan endotransglucosylase (MXE) and cellulose : xyloglucan endotransglucosylase (CXE). HTG is highly acidic and found only in the evolutionarily isolated genus of fern-allies, Equisetum. We now report genes for three new highly acidic HTG-related XTHs in E. fluviatile (EfXTH-A, EfXTH-H and EfXTH-I). We expressed them heterologously in Pichia and tested the encoded proteins' enzymic activities to determine whether their acidity and/or their Equisetum-specific sequences might confer high hetero-transglucanase activity. Untransformed Pichia was found to secrete MLG-degrading enzyme(s), which had to be removed for reliable MXE assays. All three acidic EfXTHs exhibited very predominantly XET activity, although low but measurable hetero-transglucanase activities (MXE and CXE) were also detected in EfXTH-H and EfXTH-I. We conclude that the extremely high hetero-transglucanase activities of Equisetum HTG are not emulated by similarly acidic Equisetum XTHs that share up to 55.5% sequence identity with HTG.
转糖基酶是一种能够重塑植物初生细胞壁的酶，它可能会使细胞壁变松或变强。木葡聚糖内转糖基酶（XET；EC 2.4.1.207）在陆生植物中普遍存在，是一种同型转葡聚糖酶活性（供体，木葡聚糖；受体，木葡聚糖），由 XTH（木葡聚糖内转糖基酶/水解酶）蛋白表现出来。相比之下，异型-β-葡聚糖酶（HTG）是唯一已知的优先表现为异型转葡聚糖酶的酶。它的两种主要异型转葡聚糖酶活性是 MLG：木葡聚糖内转糖基酶（MXE）和纤维素：木葡聚糖内转糖基酶（CXE）。HTG 高度酸性，仅存在于进化上孤立的木贼属植物中。我们现在报告在 E. fluviatile（EfXTH-A、EfXTH-H 和 EfXTH-I）中有三个新的高度酸性 HTG 相关 XTH 的基因。我们在毕赤酵母中异源表达了它们，并测试了编码蛋白的酶活性，以确定它们的酸性和/或它们在木贼属植物中的特异性序列是否可能赋予高异型转葡聚糖酶活性。未转化的毕赤酵母被发现分泌 MLG 降解酶，这些酶必须被去除，才能进行可靠的 MXE 测定。三种酸性 EfXTH 都表现出非常显著的 XET 活性，尽管 EfXTH-H 和 EfXTH-I 也检测到低但可测量的异型转葡聚糖酶活性（MXE 和 CXE）。我们得出结论，与 HTG 具有高达 55.5%序列同一性的类似酸性木贼属 XTH 并不能模拟木贼属 HTG 极高的异型转葡聚糖酶活性。
