Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic ‘chaperones’ can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb2+ and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro. Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development.

中文翻译：

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拟南芥阿拉伯半乳聚糖蛋白 (AGP31) 和几个阳离子 AGP 片段催化鼠李糖半乳聚糖-II 的硼桥接

鼠李糖半乳糖醛酸-II (RG-II) 是植物初级细胞壁中的一个复杂的果胶结构域。在体内，大多数 RG-II 结构域通过硼酸二酯桥共价二聚化，这对于正确的细胞壁组装至关重要，但在体外，纯 RG-II 单体通过硼酸的二聚化非常缓慢。阳离子“伴侣”可以促进二聚化，可能是通过克服相邻阴离子 RG-II 分子之间的相互排斥。高效的人工伴侣包括 Pb2+ 和多组氨酸，但提出的天然伴侣仍然难以捉摸。我们现在已经测试了几种拟南芥阿拉伯半乳聚糖蛋白 (AGP) 作为候选的阳离子肽片段。AGP17、18、19 和 31 的片段是有效的，通常在 ∼25 µg/ml (9–19 µM) 时，在体外促进 16–20 µM 单体 RG-II 在 pH 4.8 下的硼桥接。天然 AGP31 糖蛋白也有效，六组氨酸效果适中。所有测试的分子伴侣都与 RG-II 可逆地相互作用，并且在反应过程中不被消耗；因此它们起催化作用，并且可能构成第一个报道的硼作用酶活性，即RG-II硼酸二酯酶。许多肽伴侣在较高浓度下成为不太有效的催化剂，我们将其解释为由于形成了具有净正电荷的 RG-II-肽复合物，与带负电的纯 RG-II 分子一样相互排斥。这里研究的四种独特的 AGP 可能在活植物细胞中发挥酶作用，作用于高尔基池内的 RG-II 和/或分泌后的质外体。通过这种方式，RG-II 和特定的 AGP 可能有助于细胞壁组装，从而促进植物细胞的扩增和发育。