Adams Scott V, DeFelice Louis J
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37232, USA.
Biophys J. 2003 Sep;85(3):1548-59. doi: 10.1016/S0006-3495(03)74587-1.
We have investigated the conduction states of human serotonin transporter (hSERT) using the voltage clamp, cut-open frog oocyte method under different internal and external ionic conditions. Our data indicate discrepancies in the alternating access model of cotransport, which cannot consistently explain substrate transport and electrophysiological data. We are able simultaneously to isolate distinct external and internal binding sites for substrate, which exert different effects upon currents conducted by hSERT, in contradiction to the alternating access model. External binding sites of coupled Na ions are likewise simultaneously accessible from the internal and external face. Although Na and Cl are putatively cotransported, they have opposite effects on the internal face of the transporter. Finally, the internal K ion does not compete with internal 5-hydroxytryptamine for empty transporters. These data can be explained more readily in the language of ion channels, rather than carrier models distinguished by alternating access mechanisms: in a channel model of coupled transport, the currents represent different states of the same permeation path through hSERT and coupling occurs in a common pore.
我们采用电压钳制、切开式蛙卵母细胞法，在不同的内外离子条件下研究了人类血清素转运体（hSERT）的传导状态。我们的数据表明，共转运的交替访问模型存在差异，该模型无法始终如一地解释底物转运和电生理数据。与交替访问模型相反，我们能够同时分离出不同的底物外部和内部结合位点，这些位点对hSERT传导的电流有不同影响。耦合的钠离子的外部结合位点同样可从内部和外部表面同时访问。虽然钠和氯被认为是共转运的，但它们对转运体的内表面有相反的影响。最后，内部钾离子不会与内部5-羟色胺竞争空的转运体。这些数据用离子通道的语言比用以交替访问机制为特征的载体模型更容易解释：在耦合转运的通道模型中，电流代表通过hSERT的同一渗透路径的不同状态，耦合发生在一个共同的孔中。
