Parsons S M
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA.
FASEB J. 2000 Dec;14(15):2423-34. doi: 10.1096/fj.00-0203rev.
Sequence-related vesicular acetylcholine transporter (VAChT) and vesicular monoamine transporter (VMAT) transport neurotransmitter substrates into secretory vesicles. This review seeks to identify shared and differentiated aspects of the transport mechanisms. VAChT and VMAT exchange two protons per substrate molecule with very similar initial velocity kinetics and pH dependencies. However, vesicular gradients of ACh in vivo are much smaller than the driving force for uptake and vesicular gradients of monoamines, suggesting the existence of a regulatory mechanism in ACh storage not found in monoamine storage. The importance of microscopic rather than macroscopic kinetics in structure-function analysis is described. Transporter regions affecting binding or translocation of substrates, inhibitors, and protons have been found with photoaffinity labeling, chimeras, and single-site mutations. VAChT and VMAT exhibit partial structural and mechanistic homology with lactose permease, which belongs to the same sequence-defined superfamily, despite opposite directions of substrate transport. The vesicular transporters translocate the first proton using homologous aspartates in putative transmembrane domain X (ten), but they translocate the second proton using unknown residues that might not be conserved between them. Comparative analysis of the VAChT and VMAT transport mechanisms will aid understanding of regulation in neurotransmitter storage.
序列相关的囊泡乙酰胆碱转运体（VAChT）和囊泡单胺转运体（VMAT）将神经递质底物转运到分泌囊泡中。本综述旨在确定转运机制的共同和不同方面。VAChT和VMAT每底物分子交换两个质子，具有非常相似的初始速度动力学和pH依赖性。然而，体内乙酰胆碱的囊泡梯度远小于摄取的驱动力和单胺的囊泡梯度，这表明乙酰胆碱储存中存在单胺储存中未发现的调节机制。描述了微观而非宏观动力学在结构-功能分析中的重要性。通过光亲和标记、嵌合体和单点突变发现了影响底物、抑制剂和质子结合或转运的转运体区域。尽管底物转运方向相反，但VAChT和VMAT与乳糖通透酶具有部分结构和机制同源性，乳糖通透酶属于同一序列定义的超家族。囊泡转运体使用推定的跨膜结构域X（十）中的同源天冬氨酸转运第一个质子，但它们使用可能在它们之间不保守的未知残基转运第二个质子。对VAChT和VMAT转运机制的比较分析将有助于理解神经递质储存中的调节。
