植物生長素(Auxin)是控制植物生長和發育的重要荷爾蒙。 生長素轉運蛋白(AUX1)是第一個被鑑定且廣泛研究的生長素運輸蛋白，對於根的向地作用及側根和根毛的發育有著重要的作用。然而，對於此生長素轉運蛋白的生物化學性質仍然不是很清楚。在本研究中，我們測試了金屬離子對生長素轉運蛋白的離子效應，並發現當環境中存在鉀離子時，生長素的轉運效率會增強4倍。接著進一步的實驗顯示，氟化鉀活化生長素轉運蛋白的運輸效率高於氯化鉀，溴化鉀和碘化鉀。此外鉀離子和生長素轉運蛋白之間的相互作用賦予生長素轉運蛋白更高的耐熱性質，但卻更容易受到蛋白質水解。利用化學修飾藥物處理結果顯示生長素轉運蛋白的細胞外酸性氨基酸在鉀離子的活化效應中起關鍵作用。定點突變結果顯示生長素轉運蛋白的Ｄ166，Ｄ293和Ｄ312由天門冬胺酸酸置換成丙胺酸會抑制了鉀離子活化效應。相反，當這些位置突變為麩胺酸、精胺酸與天門烯胺酸，只有D312E突變株其生長素轉運活性恢復至接近野生型。因此可以推測D312可能是鉀離子活化相關胺基酸中相對重要的位點。

Auxin regulates diverse processes involved in plant growth and development. AUX1 is the first identified and most widely investigated auxin importer, and plays an important role in root gravitropism and the development of lateral root and root hair. However, the regulation of auxin transport by AUX1 is still not well understood. In this study, we examined the effect of metal ions on AUX1 transport function and found that the activity could be specifically stimulated four times by K+. Further experiments revealed the preference of KF on the enhancement of transport activity of AUX1 over KCl, KBr, and KI. In addition, the interaction between K+ and AUX1 confers AUX1 more resistant to thermal stress but more vulnerable to proteolysis. Conventional chemical modification indicated that the extracellular acidic amino acids of AUX1 play a key role in the K+ stimulation. Site-specific mutagenesis showed that the replacement of Asp166, Asp293, and Asp312 of AUX1 to alanine deteriorated the K+-stimulated auxin transport. By contrast, when these residues were mutated to glutamate, lysine or asparagine, only the D312E variant restored the IAA transport activity to the wild-type level. It is thus convinced that D312 is the most promising residue for the K+ stimulation on AUX1.

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