Transmembrane Reorientation of the Substrate-Binding Site in Vesicular Acetylcholine Transporter

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• 作者：Dawn T. Bravo ; Natalia G. Kolmakova ; and Stanley M. Parsons
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：July 13, 2004
• 年：2004
• 卷：43
• 期：27
• 页码：8787 - 8793
• 全文大小：90K
• 年卷期：v.43,no.27(July 13, 2004)
• ISSN：1520-4995

文摘

Active transport of acetylcholine (ACh) by vesicular ACh transporter (VAChT) is driven bya proton-motive force established by V-ATPase. A published microscopic kinetics model predicts theACh-binding site is primarily oriented toward the outside for nontransporting VAChT and toward theinside for transporting VAChT. The allosteric ligand [³H]vesamicol cannot bind when the ACh-bindingsite is outwardly oriented and occupied by ACh, but it can bind when the ACh site is inwardly oriented.The kinetics model was tested in the paper reported here using rat VAChT expressed in PC12^A1237 cells.Equilibrium titrations of [³H]vesamicol binding and ACh competition show that ATP blocks competitionbetween vesamicol and ACh in over one-half of the VAChT. NaCl did not mimic ACh chloride, andbafilomycin A₁ and FCCP completely blocked the ATP effect, which shows that it is mediated by aproton-motive force. The data are consistent with reorientation of over one-half of the ACh-binding sitesfrom the outside to the inside of vesicles upon activation of transport. The observations support the proposedmicroscopic kinetics model, and they should be useful in characterizing effects of mutations on the VAChTtransport cycle.