Maier R, Scholz C, Schmid F X
Laboratorium für Biochemie, Universität Bayreuth, D-95440 Bayreuth, Germany.
J Mol Biol. 2001 Dec 14;314(5):1181-90. doi: 10.1006/jmbi.2000.5192.
Trigger factor is a ribosome-bound folding helper, which, apparently, combines two functions, chaperoning of nascent proteins and catalyzing prolyl isomerization in their folding. Immediate chaperone binding at the ribosome might interfere with rapid protein folding reactions, and we find that trigger factor indeed retards the in vitro folding of a protein with native prolyl isomers. The kinetic analysis of trigger factor binding to a refolding protein reveals that the adverse effects of trigger factor on conformational folding are minimized by rapid binding and release. The complex between trigger factor and a substrate protein is thus very short-lived, and fast-folding proteins can escape efficiently from an accidental interaction with trigger factor. Protein chains with incorrect prolyl isomers cannot complete folding and therefore can rebind for further rounds of catalysis. Unlike DnaK, trigger factor interacts with substrate proteins in a nucleotide-independent binding reaction, which seems to be optimized for high catalytic activity rather than for chaperone function. The synthetic lethality, observed when the genes for both DnaK and trigger factor are disrupted, might result from an indirect linkage. In the absence of trigger factor, folding is retarded and more aggregates form, which can neither be prevented nor disposed of when DnaK is lacking as well.
触发因子是一种与核糖体结合的折叠辅助蛋白，显然它兼具两种功能，即陪伴新生蛋白质折叠以及在其折叠过程中催化脯氨酰异构化。伴侣蛋白在核糖体上的即时结合可能会干扰蛋白质的快速折叠反应，并且我们发现触发因子确实会延缓具有天然脯氨酰异构体的蛋白质的体外折叠。对触发因子与重折叠蛋白结合的动力学分析表明，触发因子对构象折叠的不利影响可通过快速结合和释放而减至最小。因此，触发因子与底物蛋白之间的复合物寿命极短，快速折叠的蛋白质能够有效地避免与触发因子发生偶然的相互作用。具有不正确脯氨酰异构体的蛋白质链无法完成折叠，因此可以重新结合以进行进一步的催化循环。与DnaK不同，触发因子在不依赖核苷酸的结合反应中与底物蛋白相互作用，这种反应似乎是为了实现高催化活性而非伴侣功能而优化的。当DnaK和触发因子的基因均被破坏时所观察到的合成致死性，可能是由间接联系导致的。在没有触发因子的情况下，折叠会受到阻碍，并且会形成更多的聚集体，而当同时缺乏DnaK时，这些聚集体既无法被阻止也无法被清除。
