Protein degradation in Escherichia coli. I. Measurement of rapidly and slowly decaying components.
Protein degradation in Escherichia coli. I. Measurement of rapidly and slowly decaying components.
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发表时间：
1970-06
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通讯作者：
K. Nath;A. L. Koch
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作者：
K. Nath;A. L. Koch
摘要通过含载体灌流液中放射性的出现，直接测量了膜过滤器支持的细菌细胞内放射性蛋白的降解率。为此，已经用微型注射器过滤器支架构造了一种灌注器。该装置内的条件是生理性的，并允许指数增长到1x108个细胞。胞外载体亮氨酸-12C与胞内亮氨酸-14C池的交换是如此之快，以至于至少87%的蛋白质分解产生的氨基酸出现在灌流液中，即使在生长中的培养物中也没有被回收到蛋白质中。只有一小部分细胞蛋白质会被快速降解。在各种营养条件下(包括生长和饥饿)，它的半衰期约为1小时，占生长在葡萄糖最低浓度的细胞总蛋白的2%~7%。与这种快速降解的蛋白质组分的合成有关的总蛋白质合成的比例随着生长速度的降低而增加。在非常慢的生长速度下，结合到蛋白质中的放射性的10%到40%被结合到快速降解的蛋白质中。去除快速成分腐烂前后细胞的非蛋白质物质和Pronase消化的处理证实了快速腐烂成分来自细菌蛋白质的结论。在灌流液中还有另一种放射性成分，只要进行实验(48小时)，就会以每小时0.2%至0.6%的非常缓慢的速度释放。这并不代表细胞内蛋白质降解为氨基酸，因为这一过程独立于亮氨酸交换，而且灌流液中30%至50%的放射性是酸不溶的。
Abstract The rate of degradation of intracellular radioactive protein of bacterial cells supported by a membrane filter has been measured directly from the appearance of radioactivity in the carrier-containing perfusate. For this purpose a perfusion apparatus has been constructed from a microsyringe filter holder. The conditions inside the apparatus are physiological and permit exponential growth up to 1 x 108 cells. Exchange of extracellular carrier leucine-12C with the intracellular leucine-14C pool is so rapid that at least 87% of the amino acid arising from protein breakdown appears in the perfusate and is not recycled into protein even in growing cultures. Only a limited portion of the cellular protein is subject to rapid degradation. It decays with a half-life of approximately 1 hour and constitutes 2 to 7% of the total cellular protein of cells growing in glucose minimal medium under various nutritional conditions (including growth and starvation). The proportion of the total protein synthesis which is directed to the synthesis of this rapidly degrading protein component increases with decreasing growth rate. At very slow growth rates 10 to 40% of the radioactivity that is incorporated into protein is incorporated into the rapidly degrading protein. Treatment which removes nonproteinaceous material and Pronase digestion of cells before and after the decay of the rapid component substantiates the conclusion that the rapidly decaying component is derived from bacterial protein. There is another radioactive component in the perfusate which is released at the very slow rate of 0.2 to 0.6% per hour for as long as the experiments are carried out (48 hours). This does not represent degradation of intracellular protein to amino acids since this process is independent of leucine exchange, and 30 to 50% of the radioactivity in the perfusate is acid-insoluble.