Bernstein Geula M, Jones Owen T
Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network. 399 Bathurst Street, Toronto, Ontario, Canada
Cell Calcium. 2007 Jan;41(1):27-40. doi: 10.1016/j.ceca.2006.04.010. Epub 2006 Jun 8.
The contribution of voltage-gated calcium channels to excitable cell function depends, critically, upon the mechanisms that control their expression at the cell surface. While co-assembly of the pore forming alpha(1) and auxiliary beta subunits enhances channel surface expression, the levels are still only 30-40% of those seen with the core alpha(1B)/beta(1b)/alpha(2)delta calcium channel complex. To rationalize this observation, it has been suggested that the alpha(2)/delta subunit might stabilize calcium channel expression at the cell surface. To test this notion, we have resolved the effect of the alpha(2)/delta subunit on the rates of binding, internalization and degradation of defined N-type calcium channel surface complexes expressed in HEK293 cells, through pulse-labeling with the selective, cell impermeable, radioligand [(125)I]-omega-CgTx. Through detailed kinetic and sensitivity analysis we show that alpha(1B)/beta(1b)/alpha(2)delta complexes are internalized slowly (k(int) 0.4/h), whereupon, most become degraded (k(deg) 0.02/h). In contrast, alpha(1B)/beta(1b) complexes are internalized more rapidly (k(int) 0.8/h), following which they are either quickly degraded (k(deg) 0.1/h) or are sequestered slowly (k(tra) 0.1/h) to a pool that is metabolically stable within the time-frame of our experiments (24h). In neither case did we find evidence for recycling via the cell surface. Thus, our data argue for a novel mechanism where complexes lacking an alpha(2)/delta subunit are cleared from the cell surface and are rapidly degraded or stored, possibly for further attempts at complexation as new alpha(2)/delta subunits become available. The slower rate of internalization of complexes containing the alpha(2)/delta subunit rationalizes the stabilizing effect this subunit has upon calcium channel surface expression and suggests a mechanism by which alpha(2)delta mutations may cause severe neurological deficits.
电压门控钙通道对可兴奋细胞功能的贡献，关键取决于控制其在细胞表面表达的机制。虽然形成孔道的α(1)亚基与辅助β亚基的共同组装可增强通道在表面的表达，但表达水平仍仅为核心α(1B)/β(1b)/α(2)δ钙通道复合物的30 - 40%。为解释这一现象，有人提出α(2)/δ亚基可能稳定钙通道在细胞表面的表达。为验证这一观点，我们通过用选择性、细胞不可渗透的放射性配体[(125)I]-ω-芋螺毒素进行脉冲标记，解析了α(2)/δ亚基对在HEK293细胞中表达的特定N型钙通道表面复合物的结合、内化和降解速率的影响。通过详细的动力学和敏感性分析，我们发现α(1B)/β(1b)/α(2)δ复合物内化缓慢（内化速率常数k(int)为0.4/小时），随后大部分被降解（降解速率常数k(deg)为0.02/小时）。相比之下，α(1B)/β(1b)复合物内化更快（k(int)为0.8/小时），之后它们要么迅速被降解（k(deg)为0.1/小时），要么缓慢被隔离（转运速率常数k(tra)为0.1/小时）到一个在我们实验时间框架（24小时）内代谢稳定的池子里。在这两种情况下，我们都没有找到通过细胞表面循环利用的证据。因此，我们的数据支持一种新机制，即缺乏α(2)/δ亚基的复合物从细胞表面清除并迅速降解或储存，可能是为了在新的α(2)/δ亚基可用时进一步尝试形成复合物。含有α(2)/δ亚基的复合物内化速率较慢，解释了该亚基对钙通道表面表达的稳定作用，并提示了α(2)δ突变可能导致严重神经功能缺损所涉及的机制。
