Weise Alexandra, Becker Holger M, Deitmer Joachim W
Abteilung für Allgemeine Zoologie, FB Biologie, Universitaet Kaiserslautern, D-67653 Kaiserslautern, Germany.
J Gen Physiol. 2007 Aug;130(2):203-15. doi: 10.1085/jgp.200709809.
The transport activity of the glutamine/neutral amino acid transporter SNAT3 (former SN1, SLC38A3), expressed in oocytes of the frog Xenopus laevis is associated with a non-stoichiometrical membrane conductance selective for Na(+) and/or H(+) (Schneider, H.P., S. Bröer, A. Bröer, and J.W. Deitmer. 2007. J. Biol. Chem. 282:3788-3798). When we expressed SNAT3 in frog oocytes, the glutamine-induced membrane conductance was suppressed, when carbonic anhydrase isoform II (CAII) had been injected into the oocytes. Transport of substrate, however, was not affected by CAII. The reduction of the membrane conductance by CAII was dependent on the presence of CO(2)/HCO(3)(-), and could be reversed by blocking the catalytic activity of CAII by ethoxyzolamide (10 microM). Coexpression of wild-type CAII or a N-terminal CAII mutant with SNAT3 also reduced the SNAT3- associated membrane conductance. The catalytically inactive CAII mutant V143Y coexpressed in oocytes did not affect SNAT3-associated membrane conductance. Our results reveal a new type of interaction between CAII and a transporter-associated cation conductance, and support the hypothesis that the transport of substrate and the non-stoichiometrical ion conductance are independent of each other. This study also emphasizes the importance of carbonic anhydrase activity and the presence of CO(2)-bicarbonate buffers for membrane transport processes.
谷氨酰胺/中性氨基酸转运体SNAT3（原SN1，SLC38A3）在非洲爪蟾卵母细胞中表达，其转运活性与对Na(+)和/或H(+)具有选择性的非化学计量膜电导相关（施奈德，H.P.，S. 布勒尔，A. 布勒尔，和J.W. 戴特默。2007年。《生物化学杂志》282:3788 - 3798）。当我们在爪蟾卵母细胞中表达SNAT3时，如果将碳酸酐酶同工型II（CAII）注入卵母细胞，谷氨酰胺诱导的膜电导会受到抑制。然而，底物的转运不受CAII影响。CAII对膜电导的降低依赖于CO(2)/HCO(3)(-)的存在，并且可以通过用乙氧唑胺（10微摩尔）阻断CAII的催化活性来逆转。野生型CAII或N端CAII突变体与SNAT3共表达也会降低与SNAT3相关的膜电导。在卵母细胞中共表达的无催化活性的CAII突变体V143Y不影响与SNAT3相关的膜电导。我们的结果揭示了CAII与转运体相关阳离子电导之间的一种新型相互作用，并支持底物转运和非化学计量离子电导相互独立的假说。这项研究还强调了碳酸酐酶活性以及CO(2)-碳酸氢盐缓冲液对膜转运过程的重要性。
