Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Mol Microbiol. 2010 Mar;75(6):1539-49. doi: 10.1111/j.1365-2958.2010.07070.x. Epub 2010 Feb 10.
Small heat-shock proteins (sHSPs) are a widely conserved family of molecular chaperones, all containing a conserved alpha-crystallin domain flanked by variable N- and C-terminal tails. We report that IbpA and IbpB, the sHSPs of Escherichia coli, are substrates for the AAA+ Lon protease. This ATP-fueled enzyme degraded purified IbpA substantially more slowly than purified IbpB, and we demonstrate that this disparity is a consequence of differences in maximal Lon degradation rates and not in substrate affinity. Interestingly, however, IbpB stimulated Lon degradation of IbpA both in vitro and in vivo. Furthermore, although the variable N- and C-terminal tails of the Ibps were dispensable for proteolytic recognition, these tails contain critical determinants that control the maximal rate of Lon degradation. Finally, we show that E. coli Lon degrades variants of human alpha-crystallin, indicating that Lon recognizes conserved determinants in the folded alpha-crystallin domain itself. These results suggest a novel mode for Lon substrate recognition and provide a highly suggestive link between the degradation and sHSP branches of the protein quality-control network.
小分子热休克蛋白(sHSPs)是一类广泛保守的分子伴侣家族,均含有保守的α-晶状体蛋白结构域,两侧为可变的 N-端和 C-端尾部。我们报告称,大肠杆菌的 sHSPs,即 IbpA 和 IbpB,是 AAA+Lon 蛋白酶的底物。这种由 ATP 驱动的酶对纯化的 IbpA 的降解速度明显慢于对纯化的 IbpB 的降解速度,我们证明这种差异是由于 Lon 最大降解速率的差异,而不是由于底物亲和力的差异。然而,有趣的是,IbpB 无论是在体外还是在体内,均能刺激 Lon 对 IbpA 的降解。此外,尽管 Ibp 的可变 N-端和 C-端尾部对于蛋白水解识别不是必需的,但这些尾部包含控制 Lon 最大降解速率的关键决定因素。最后,我们表明大肠杆菌 Lon 可以降解人α-晶状体蛋白的变体,表明 Lon 可以识别折叠的α-晶状体蛋白结构域本身的保守决定因素。这些结果提出了 Lon 底物识别的一种新方式,并在蛋白质质量控制网络的降解和 sHSP 分支之间提供了高度提示性的联系。