主题：Co-translational Protein Folding, One Molecule at a Time
Protein folding can begin co-translationally. Due to the difference in timescale between folding and synthesis, co-translational folding is thought to occur at equilibrium for fast folding domains. Thus, the folding kinetics of stalled ribosome-bound nascent chains should match the folding of nascent chains in real time. We test this assumption by comparing the folding of a ribosome-bound, multi-domain calcium-binding protein stalled at different points in translation with the nascent chain as is it being synthesized in real-time, via optical tweezers. In vitro, a misfolded state of the protein occurs readily, and on stalled ribosomes, the misfolded state still forms rapidly (1.5 s). Surprisingly, during active translation, this state is only attained after a long delay (~ 60 s), indicating that, unexpectedly, the growing polypeptide is not equilibrated with its ensemble of accessible conformations. Slow equilibration on the ribosome can delay premature folding until adequate sequence is available and/or allow time for chaperone binding, thus promoting productive folding. On the other hand, interactions between the nascent polypeptide and the ribosome exit tunnel represent one mode of regulating synthesis rates, which, in turn, are thought to affect protein folding. The SecM protein arrests its own translation as part of a feedback mechanism, and release of arrest has been proposed to occur by mechanical force at the translocon. This is the so-called translocon mechanical hypothesis. Using optical tweezers, I demonstrate that arrest of SecM-stalled ribosomes can indeed be rescued by force alone. Moreover, I will show that the force needed to release stalling can be generated in vivo by a nascent chain folding near the ribosome tunnel exit. I formulate a kinetic model describing how a protein can regulate its own synthesis by the force generated during folding, tuning ribosome activity to structure acquisition by a nascent polypeptide.
Bustamante教授是世界著名的生物物理学家，是多领域交叉学科研究的代表性人物。他利用单分子可视化方法，在世界上首次测定了单根DNA分子的弹性，开启了生物单分子力学研究的热潮。Bustamante教授于1981年获美国加州大学伯克利分校生物物理学博士学位，随后在该校的Lawrence Berkeley实验室从事博士后研究工作。1998年起担任美国加州大学伯克利分校分子与细胞生物学、物理学与化学教授，2000年起担任霍华德休斯医学研究所研究员。迄今已在国际一流学术杂志中发表文章300余篇，其中在Science、Nature、Cell等顶级杂志上发表文章达50余篇。他荣获了众多奖项，包括于2004年获得Alexander Hollaender生物物理学奖，2012年获得维尔切克奖，2021 年获得Kazuhiko Kinosita Award。