Sec1/Munc18 (SM) proteins contribute to membrane fusion by interacting with Qa-SNAREs or nascent trans-SNARE complexes. Gymnosperms and the basal angiosperm Amborella have only a single SEC1 gene related to the KEULE gene in Arabidopsis However, the genomes of most angiosperms including Arabidopsis encode three SEC1-related SM proteins of which only KEULE has been functionally characterized as interacting with the cytokinesis-specific Qa-SNARE KNOLLE during cell-plate formation. Here we analyze the closest paralog of KEULE named SEC1B. In contrast to the cytokinesis defects of keule mutants, sec1b mutants are homozygous viable. However, the keule sec1b double mutant was nearly gametophytically lethal, displaying collapsed pollen grains, which suggests substantial overlap between SEC1B and KEULE functions in secretion-dependent growth. SEC1B had a strong preference for interaction with the evolutionarily ancient Qa-SNARE SYP132 involved in secretion and cytokinesis, whereas KEULE interacted with both KNOLLE and SYP132. This differential interaction with Qa-SNAREs is likely conferred by domains 1 and 2a of the two SM proteins. Comparative analysis of all four possible combinations of the relevant SEC1 Qa-SNARE double mutants revealed that in cytokinesis, the interaction of SEC1B with KNOLLE plays no role, whereas the interaction of KEULE with KNOLLE is prevalent and functionally as important as the interactions of both SEC1B and KEU with SYP132 together. Our results suggest that functional diversification of the two SEC1-related SM proteins during angiosperm evolution resulted in enhanced interaction of SEC1B with Qa-SNARE SYP132, and thus a predominant role of SEC1B in secretion.

中文翻译：

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拟南芥SEC1相关SM蛋白在细胞动力学和分泌膜融合中的功能多样化。

Sec1 / Munc18（SM）蛋白通过与Qa-SNARE或新生的反式SNARE复合物相互作用而促进膜融合。裸子植物和基被子植物Amborella仅具有一个与拟南芥中KEULE基因相关的SEC1基因。然而，包括拟南芥在内的大多数被子植物的基因组均编码三种与SEC1相关的SM蛋白，其中只有KEULE在功能上被表征为与胞质分裂特异性相互作用Qa-SNARE KNOLLE在细胞板形成过程中。在这里，我们分析了KEULE的最接近的对等物，命名为SEC1B。与keule突变体的胞质分裂缺陷相反，sec1b突变体是纯合的。但是，短尾小节sec1b双重突变体几乎是致配子体致死的，显示出花粉粒塌陷，这表明SEC1B和KEULE在分泌依赖性生长中的功能之间存在实质性重叠。SEC1B非常喜欢与涉及分泌和胞质分裂的进化上古老的Qa-SNARE SYP132相互作用，而KEULE与KNOLLE和SYP132都相互作用。与Qa-SNARE的这种差异性相互作用可能是由两个SM蛋白的结构域1和2a引起的。对相关SEC1 Qa-SNARE双重突变体的所有四种可能组合的比较分析表明，在胞质分裂中，SEC1B与KNOLLE的相互作用不起作用，而KEULE与KNOLLE的相互作用普遍且在功能上与两个SEC1B的相互作用一样重要和KEU与SYP132一起使用。我们的结果表明，在被子植物进化过程中，两个与SEC1相关的SM蛋白的功能多样化导致SEC1B与Qa-SNARE SYP132的相互作用增强，