G-protein deactivation is rate-limiting for shut-off of the phototransduction cascade
G-protein deactivation is rate-limiting for shut-off of the phototransduction cascade
复制标题
DOI：
10.1038/38750
复制
发表时间：
1997-09-25
期刊：
影响因子：
64.8
通讯作者：
Lagnado, L
中科院分区：
文献类型：
--
作者：
Sagoo, MS;Lagnado, L
光感受器通过七螺旋受体（视紫红质）和与环GMP磷酸二酯酶偶联的异源三聚体G蛋白（转导素）检测光（1，2）。类似的途径用于放大对激素，味觉和嗅觉的反应（3 - 5）。胞质Ca 2+的下降降低了光转导的放大（参考文献6 - 10），但尚不清楚视紫红质和转导蛋白的失活如何影响这种反应，从而影响磷酸二酯酶活性的程度和持续时间（11 - 14）。在这里，我们通过记录截短的视杆光感受器对闪光的电响应来研究这一点（10）。通过去除ATP以阻止视紫红质的磷酸化失活（15），我们表明该反应限制了响应的幅度，并且在含有1 μ M Ca2+的溶液中闪光的3.2 s内开始，在零Ca2+溶液中下降到0.9 s。相比之下，激活和振幅的响应不受影响时，转导蛋白失活GTP水解被阻断取代GTP与其nonhydrolysable类似物GTP-γ S-11，表明有一点GTP水解发生在光激发视紫红质被淬灭的期间。因此，视紫红质的快速失活是控制光响应幅度的Ca2+敏感步骤，而转导蛋白失活较慢并控制恢复。
Photoreceptors detect light through a seven-helix receptor (rhodopsin) and heterotrimeric G protein (transducin) coupled to a cyclic GMP phosphodiesterase(1,2). Similar pathways are used to amplify responses to hormones, taste and smell(3-5). The amplification of phototransduction is reduced by a fall in cytoplasmic Ca2+ (refs 6-10), but it is not known how the deactivation of rhodopsin and transducin influence this response and hence the extent and duration of phosphodiesterase activity(11-14). Here we investigate this by recording the electrical response to flashes of light in truncated rod photoreceptors(10). By removing ATP to block the deactivation of rhodopsin by phosphorylation(15), we show that this reaction limits the amplitude of the response and begins within 3.2 s of a flash in a solution containing 1 mu M Ca2+, falling to 0.9 s in a zero-Ca2+ solution. In contrast, the activation and amplitude of the response were unaffected when transducin deactivation by GTP hydrolysis was blocked by replacing GTP with its nonhydrolysable analogue GTP-gamma S-11, demonstrating that there is little GTP hydrolysis occurring over the period in which photoexcited rhodopsin is quenched. The rapid deactivation of rhodopsin is therefore a Ca2+-sensitive step controlling the amplitude of the light response, whereas transducin deactivation is slower and controls recovery.