Structural implications of mutations in the pea SYM8 symbiosis gene, the DMI1 ortholog, encoding a predicted ion channel
Structural implications of mutations in the pea SYM8 symbiosis gene, the DMI1 ortholog, encoding a predicted ion channel
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DOI：
10.1094/mpmi-20-10-1183
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发表时间：
2007-10-01
影响因子：
3.5
通讯作者：
Downie, J. Allan
中科院分区：
文献类型：
--
作者：
Edwards, Anne;Heckmann, Anne B.;Downie, J. Allan
豌豆SYM8基因在根瘤菌和菌根共生中都起着重要的作用。原始类型品系R25中sym8的突变阻止了结瘤、菌丝化和结瘤因子诱导的钙尖峰，这是结瘤信号通路的早期组成部分。我们描述了豌豆的4个新的sym8等位基因，它们与R25属于同一个互补组。Sym8突变体在表型上与紫花苜蓿dmi1突变体相似，并定位于同线位置。我们利用序列同源性分离了元胞根结瘤菌DMI1的豌豆直系同源基因，并证明克隆的豌豆同源基因能够互补元胞根结瘤突变株dmil的结瘤。五个豌豆sym8突变体中的每一个都带有DMI1同源基因的突变，证实豌豆SYM8是DMI1同源基因。根据预测的与古细菌离子通道的结构相似性，我们认为SYM8形成了一个四聚体钙门控通道，其预测结构类似于古细菌钾通道，但包含不同的过滤器区域。预测的结构确定了形成通道开口的四个天冬氨酸残基(每个亚基一个)。我们进行了一个突变，将天冬氨酸改变为Valine，并在这个预测的过滤器区域发现了一个错义突变(将天冬氨酸残基附近的丙氨酸改变为Valine)；这两个突变都导致了功能丧失。我们还在一个连接预测通道和门控环域的结构域中发现了一个功能丧失的错义突变(将精氨酸改为异亮氨酸)，表明这种突变可能通过阻止蛋白质构象变化从门环结构域传递到孔道域来阻止功能。
The Pisum sativum SYM8 gene plays an essential part in both rhizobial and mycorrhizal symbioses. Mutation of sym8 in the original type line R25 blocks nodulation, mycorrhization, and Nod-factor-induced calcium spiking, an early component of the nodulation signaling pathway. We describe four new sym8 alleles of pea, which fall into the same complementation group as R25. The sym8 mutants are phenotypically similar to Medicago truncatula dmi1 mutants and map to a syntenic location. We used sequence homology to isolate the pea ortholog of M. truncatula DMI1 and have shown that the cloned pea ortholog can complement a M. truncatula dmil mutant for nodulation. Each of the five pea sym8 mutants carries a mutation in the DMI1 ortholog, confirming that the pea SYM8 is the DMI1 ortholog. Based on predicted structural similarities with an archaebacterial ion channel, we propose that SYM8 forms a tetrameric calcium-gated channel of a predicted structure similar to the archaebacterial potassium channel but containing a filter region that is different. The predicted structure identifies four aspartate residues (one from each subunit) forming the channel opening. We made a mutation changing the aspartate to valine and identified a missense mutation (changing alanine to valine adjacent to the aspartate residues) in this predicted filter region; both mutations caused a loss of function. We also identified a loss-of-function missense mutation (changing arginine to isoleucine) in a domain proposed to link the predicted channel and the gating ring domains, indicating that this mutation may block function by preventing a protein conformational change being transmitted from the gating-ring domain to the pore domain.