Bennett G S, Hollander B A, Laskowska D, DiLullo C
Department of Anatomy and Cell Biology, University of Florida, Gainesville 32610.
J Neurochem. 1991 Jul;57(1):130-9. doi: 10.1111/j.1471-4159.1991.tb02107.x.
When cultured chick sensory neurons were labeled with [35S]methionine for 1 h or longer in the presence of 5-25 mM LiCl, we found a dose-dependent reduction in the level of radiolabeled tubulin, to one third of control levels, with no noticeable effect on other proteins. The magnitude of this response was identical after a 1-h or 72-h preincubation in 25 mM LiCl and returned to control values within 1 h after removal of LiCl. Short (5-min) pulse-chase experiments revealed that tubulin synthesis was not affected by Li+, but that newly synthesized tubulin was rapidly degraded, such that 50% of the labeled beta-tubulin was lost within 5 min. There was no enhanced degradation of tubulin present before exposure to Li+. Addition of LiCl at various times before and after a 10-min pulse suggested that tubulin becomes completely refractory to Li(+)-induced degradation within 10 min after translation. Although Li+ treatment resulted in a decrease in the fraction of extant tubulin present in the unassembled form, the Li(+)-induced degradation of nascent tubulin is not a consequence of shifts in assembly state, because colcemid or taxol treatment did not lead to rapid degradation of newly synthesized tubulin, and neither drug altered the response to Li+. We suggest that Li+ interferes with the correct folding of tubulin polypeptides, exposing sites, normally hidden, to the action of a protease(s).
当在5-25 mM LiCl存在的情况下，用[35S]甲硫氨酸标记培养的鸡感觉神经元1小时或更长时间时，我们发现放射性标记的微管蛋白水平呈剂量依赖性降低，降至对照水平的三分之一，而对其他蛋白质没有明显影响。在25 mM LiCl中预孵育1小时或72小时后，这种反应的幅度相同，并且在去除LiCl后1小时内恢复到对照值。短时间（5分钟）的脉冲追踪实验表明，微管蛋白的合成不受Li +的影响，但新合成的微管蛋白迅速降解，以至于5分钟内50%的标记β-微管蛋白丢失。暴露于Li +之前存在的微管蛋白没有增强的降解。在10分钟脉冲前后的不同时间添加LiCl表明，微管蛋白在翻译后10分钟内对Li +诱导的降解完全失去反应。尽管Li +处理导致未组装形式的现存微管蛋白比例下降，但Li +诱导的新生微管蛋白降解不是组装状态改变的结果，因为秋水仙酰胺或紫杉醇处理不会导致新合成的微管蛋白迅速降解，并且这两种药物都不会改变对Li +的反应。我们认为Li +干扰了微管蛋白多肽的正确折叠，使通常隐藏的位点暴露于蛋白酶的作用之下。
