hERG为延迟整流钾电流快成分(Ikr)的基因，hERG通 道在S5跨膜片段和孔区之间有一长的细胞外环，大约39个氨基酸，这一细胞外S5-P环的点 位突变与遗传性长QT(LQT2)综合征有密切联系。因此，这一区域在决定hERG通道功能方面具 有非常重要的意义。应用先进的分子生物学方法将这一区域的两个组氨酸残基(H578和H587) 突变为脯氨酸(P)，利用青蛙卵母细胞这一表达系统将wild-type和突变的hERG通道表达于 该细胞膜，通过膜片钳技术来探讨点突变与通道功能改变之间的关系，实验研究表明在578 位置的组氨酸(H1)突变并不影响HERG通道功能，然而，在587位置的组氨酸(H2)突变就 可破坏该通道的C-型灭活过程和孔的K+选择性，通道激活的电压依赖性也向超极化侧移动 ，这一现象说明，hERG通道细胞外侧S5-P环围绕587位置肽段骨架的构象变化能够影响该通 道的门控特性和离子选择性功能。因此，该研究揭示了HERG通道S5-P环的587位置突变和LQ T2 Ikr功能丧失之间有着密切联系。%The hERG channel has a H1 long (39 amino aci ds) extracellular loop between the transmembrane S5 segment and the pore region. Point mutations in this extracellular S5-P loop have been linked to inherited long-QT (LQT2) syndrome, suggesting that this region may be important in the hE R G channel function. We explore this possibility by mutating two histidine residu es in this region(H578 and H587, referred to as H1 and H2) to various residu es and examined the resulting changes in channel function. Both positions could tolerate drastic changes in side chain properties (proline), indicating that the y are solvent exposed. None of the H1 mutation affected the hERG channel funct ion. On the other hand, putting a proline at H2 position disrupted the C-type inactivation process and the pore's K selectivity. There was also a hyperpolari zing shift in the voltage-dependence of activation. These observations suggest that peptide backbone conformation around position 587 in the extracellular S5- P loop of hERG channel can affect the channel's gating and ion selectivity functions. This may underlie the linkage between point mutations in hERG's S5-P loop and defects in Ikr function associated with LQT2.
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