The plant hormone auxin must be transported throughout plants in a cell-to-cell manner to affect its various physiological functions. ABCB transporters are critical for this polar auxin distribution, but the regulatory mechanisms controlling their function is not fully understood. The auxin transport activity of ABCB1 was suggested to be regulated by a physical interaction with FKBP42/Twisted Dwarf1 (TWD1), a peptidylprolyl cis-trans isomerase (PPIase), but all attempts to demonstrate such a PPIase activity by TWD1 have failed so far. By using a structure-based approach, we identified several surface-exposed proline residues in the nucleotide binding domain and linker of Arabidopsis ABCB1, mutations of which do not alter ABCB1 protein stability or location but do affect its transport activity. P1008 is part of a conserved signature D/E-P motif that seems to be specific for auxin-transporting ABCBs, which we now refer to as ATAs. Mutation of the acidic residue also abolishes auxin transport activity by ABCB1. All higher plant ABCBs for which auxin transport has been conclusively proven carry this conserved motif, underlining its predictive potential. Introduction of this D/E-P motif into malate importer, ABCB14, increases both its malate and its background auxin transport activity, suggesting that this motif has an impact on transport capacity. The D/E-P1008 motif is also important for ABCB1-TWD1 interactions and activation of ABCB1-mediated auxin transport by TWD1. In summary, our data imply a new function for TWD1 acting as a putative activator of ABCB-mediated auxin transport by cis-trans isomerization of peptidyl-prolyl bonds.

中文翻译：

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生长素转运 ABC 转运蛋白由受脯氨酰异构酶调节的保守 D/EP 基序定义。

植物激素生长素必须以细胞对细胞的方式在整个植物中运输，以影响其各种生理功能。ABCB 转运蛋白对于这种极性生长素分布至关重要，但控制其功能的调节机制尚不完全清楚。ABCB1 的生长素转运活性被认为是通过与 FKBP42/Twisted Dwarf1 (TWD1)、一种肽基脯氨酰顺反异构酶 (PPIase) 的物理相互作用来调节的，但到目前为止，所有通过 TWD1 证明这种 PPIase 活性的尝试都失败了。通过使用基于结构的方法，我们在拟南芥 ABCB1 的核苷酸结合域和接头中鉴定了几个暴露于表面的脯氨酸残基，其突变不会改变 ABCB1 蛋白质的稳定性或位置，但会影响其转运活性。P1008 是一个保守的特征 D/EP 基序的一部分，它似乎是生长素运输 ABCB 所特有的，我们现在将其称为 ATA。酸性残基的突变也会消除 ABCB1 的生长素转运活性。已经最终证明生长素转运的所有高等植物 ABCB 都带有这个保守的基序，突显了其预测潜力。将此 D/EP 基序引入苹果酸进口蛋白 ABCB14，可增加其苹果酸及其背景生长素运输活性，表明该基序对运输能力有影响。D/E-P1008 基序对于 ABCB1-TWD1 相互作用和 TWD1 激活 ABCB1 介导的生长素转运也很重要。总之，我们的数据暗示了 TWD1 的新功能，通过肽基 - 脯氨酰键的顺反异构化，作为 ABCB 介导的生长素转运的推定激活剂。