Silverman S K, Kofuji P, Dougherty D A, Davidson N, Lester H A
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena 91125, USA.
Proc Natl Acad Sci U S A. 1996 Dec 24;93(26):15429-34. doi: 10.1073/pnas.93.26.15429.
The homozygous weaver mouse displays neuronal degeneration in several brain regions. Previous experiments in heterologous expression systems showed that the G protein-gated inward rectifier K+ channel (GIRK2) bearing the weaver pore-region GYG-to-SYG mutation (i) is not activated by G beta gamma subunits, but instead shows constitutive activation, and (ii) is no longer a K(+)-selective channel but conducts Na+ as well. The present experiments on weaverGIRK2 (wvGIRK2) expressed in Xenopus oocytes show that the level of constitutive activation depends on intracellular Na+ concentration. In particular, manipulations that decrease intracellular Na+ produce a component of Na(+)-permeable current activated via a G protein pathway. Therefore, constitutive activation may not arise because the weaver mutation directly alters the gating transitions of the channel protein. Instead, there may be a regenerative cycle of Na+ influx through the wvGIRK2 channel, leading to additional Na+ activation. We also show that the wvGIRK2 channel is permeable to Ca2+, providing an additional mechanism for the degeneration that characterizes the weaver phenotype. We further demonstrate that the GIRK4 channel bearing the analogous weaver mutation has properties similar to those of the wvGIRK2 channel, providing a glimpse of the selective pressures that have maintained the GYG sequence in nearly all known K+ channels.
纯合的韦弗氏小鼠在几个脑区表现出神经元变性。先前在异源表达系统中的实验表明，携带韦弗氏孔区GYG到SYG突变的G蛋白门控内向整流钾通道（GIRK2）：（i）不被Gβγ亚基激活，而是表现出组成性激活；（ii）不再是K⁺选择性通道，也能传导Na⁺。目前在非洲爪蟾卵母细胞中表达的韦弗氏GIRK2（wvGIRK2）的实验表明，组成性激活的水平取决于细胞内Na⁺浓度。特别是，降低细胞内Na⁺的操作会产生通过G蛋白途径激活的Na⁺通透性电流成分。因此，组成性激活可能不是因为韦弗氏突变直接改变了通道蛋白的门控转换。相反，可能存在一个通过wvGIRK2通道的Na⁺内流的再生循环，导致额外的Na⁺激活。我们还表明，wvGIRK2通道对Ca²⁺有通透性，为表征韦弗氏表型的变性提供了另一种机制。我们进一步证明，携带类似韦弗氏突变的GIRK4通道具有与wvGIRK2通道相似的特性，这让我们得以一窥在几乎所有已知的K⁺通道中维持GYG序列的选择压力。
