Mechanism of pilus biogenesis by bacterial conjugative transfer systems

抗生素耐药性的上升是对公共卫生的主要威胁之一。几十年来，抗生素一直是细菌感染的标准护理。自发现以来，已有数以亿计的人类成功地接受了这些“神奇”药物的治疗。不幸的是，抗生素耐药性基因在细菌病原体中的快速传播可能会破坏多年来在抗击细菌感染方面取得的显著进展。接合(或接合转移)是一种广泛的生物学过程，通过这种过程，基因从细菌供体细胞水平和单向交换到细菌受体细胞。由于接合是抗生素耐药性基因在细菌病原体之间传播的过程，它在很大程度上是人类目前正在经历的抗生素耐药性危机的原因。接合转移的关键是事先形成菌毛，这是一种由供体细胞产生的中空长丝。菌毛由一种名为VirB2的蛋白质的数千个亚基组成。菌毛不仅用于与受体细胞接触，而且还作为DNA从供体细胞转移到受体细胞的管道。在DNA转移开始之前，需要生成菌毛。菌毛的生物发生和DNA转移是由一个非常大的分子机器介导的，这个机器被称为“接合转移系统”，嵌入在供体细胞膜中。虽然进行菌毛生物发生和DNA转移需要12个蛋白质，命名为VirB1-11和VirD4，但只有VirB2-11蛋白是菌毛生物发生所必需的。最近，我们已经确定了一个包含所有参与菌毛生物发生的10个蛋白质的大型复合体的冷冻-EM原子分辨结构，因此，我们能够提出一个合理的菌毛生物发生机制。在这里，我们建议在最近的工作的基础上，通过这些复杂的系统来阐明菌毛生物发生的细节。机制和结构细节将导致更好地理解抗生素耐药基因如何在细菌群体中传播，从而提供阻止它的手段。

The rise of antibiotic resistance is one of the major threats to public health. Antibiotics have been the standard of care for bacterial infections for decades. Since their discovery, hundreds of millions of humans have successfully been treated by these "wonder" drugs. Unfortunately, the rapid spread of antibiotic resistance genes among bacterial pathogens threatens to undo the remarkable progress made over the years to combat bacterial infections. Conjugation (or conjugative transfer) is a widespread biological process whereby genes are exchanged horizontally and unidirectionally from a bacterial donor cell to a bacterial recipient cell. Because conjugation is the process via which antibiotic resistance genes propagate among bacterial pathogens, it is in large part responsible for the antibiotic resistance crisis that humanity is presently experiencing. Crucial to conjugative transfer is the prior formation of a pilus, a long hollow filament produced by the donor cell. The pilus is made of 1000s of subunits of a protein called VirB2. The pilus serves not only to make contact with the recipient cell but also as a conduit for the transfer of DNAs from donor to recipient cells. A pilus needs to be generated before DNA transfer can begin. Pilus biogenesis and DNA transfer are mediated by a very large molecular machine called "conjugative transfer system" which is embedded in the membranes of the donor cell. Although 12 proteins, named VirB1-11 and VirD4, are necessary to carry out pilus biogenesis and DNA transfer, only the VirB2-11 proteins are required for pilus biogenesis. Recently, we have determined the cryo-EM atomic resolution structure of a large complex containing all the 10 proteins involved in pilus biogenesis and, as a result, we were able to propose a plausible mechanism for pilus biogenesis. Here we propose to build on this recent work and elucidate the details of the mechanism of pilus biogenesis by these complex systems. Mechanistic and structural details will lead to a better understanding of how antibiotics resistance genes spread among bacterial populations and therefore will provide the means to block it.