Beckmann R P, Lovett M, Welch W J
University of California, San Francisco, Department of Medicine 94143-0854.
J Cell Biol. 1992 Jun;117(6):1137-50. doi: 10.1083/jcb.117.6.1137.
Members of the heat-shock protein (hsp) 70 family, distributed within various cellular compartments, have been implicated in facilitating protein maturation events. In particular, related hsp 70 family members appear to bind nascent polypeptides which are in the course of synthesis and/or translocation into organelles. We previously reported that in normal, unstressed cells, cytosolic hsp 70 (hsp 72/73) interacted transiently with nascent polypeptides. We suspect that such interactions function to prevent or slow down the folding of the nascent polypeptide chain. Once synthesis is complete, and now with all of the information for folding present, the newly synthesized protein appears to commence along its folding pathway, accompanied by the ATP-dependent release of hsp 72/73. Herein, we examined how these events occur in cells subjected to different types of metabolic stress. In cells exposed to either an amino acid analog or sodium arsenite, two potent inducers of the stress response, newly synthesized proteins bind to but are not released from hsp 70. Under these conditions of metabolic stress, we suspect that the newly synthesized proteins are unable to commence proper folding and consequently remain bound to their hsp 70 chaperone. In cells subjected to heat shock, a large number of both newly synthesized as well as mature proteins are rendered insoluble. Within this insoluble material are appreciable amounts of hsp 72/73. Finally, we show that in cells depleted of ATP, the release of hsp 70 from maturing proteins is inhibited. Thus, in cells experiencing metabolic stress, newly synthesized proteins unable to properly fold, as will as mature proteins which begin to unfold become stably bound to hsp 72/73. As a consequence and over time, the free or available levels of pre-existing hsp 72/73 are reduced. We propose that this reduction in the available levels of hsp 72/73 is the trigger by which the stress response is initiated.
热休克蛋白（hsp）70家族成员分布于细胞的各个区室，与促进蛋白质成熟过程有关。特别是，相关的hsp 70家族成员似乎能结合正在合成和/或转运到细胞器中的新生多肽。我们之前报道过，在正常、未受应激的细胞中，胞质hsp 70（hsp 72/73）与新生多肽短暂相互作用。我们推测这种相互作用的功能是防止或减缓新生多肽链的折叠。一旦合成完成，此时折叠所需的所有信息都已具备，新合成的蛋白质似乎开始沿着其折叠途径进行折叠，同时伴随着hsp 72/73依赖ATP的释放。在此，我们研究了这些事件在遭受不同类型代谢应激的细胞中是如何发生的。在暴露于氨基酸类似物或亚砷酸钠（两种强效应激反应诱导剂）的细胞中，新合成的蛋白质与hsp 70结合但不会从hsp 70上释放。在这些代谢应激条件下，我们推测新合成的蛋白质无法开始正确折叠，因此仍与它们的hsp 70伴侣结合。在遭受热休克的细胞中，大量新合成的以及成熟的蛋白质变得不溶。在这种不溶物质中有相当数量的hsp 72/73。最后，我们表明在ATP耗尽的细胞中，hsp 70从正在成熟的蛋白质上的释放受到抑制。因此，在经历代谢应激的细胞中，无法正确折叠的新合成蛋白质以及开始解折叠的成熟蛋白质会与hsp 72/73稳定结合。随着时间的推移，结果是预先存在的hsp 72/73的游离或可利用水平降低。我们提出hsp 72/73可利用水平的这种降低是启动应激反应的触发因素。
