Bypass of a protein barrier by a replicative DNA helicase

摘要

“猿病毒-40”(SV40)的复制性解旋酶(大型T-抗原)(T-Ag)过去被认为形成一个“双六聚物”，后者从中间穿过双链DNA，迫使被解开的单链DNA从边上出去。现在，Johannes Walter及其同事发现，这个模型是不正确的，因为功能性形式是个“单六聚物”。令人吃惊的是，结合到DNA上的蛋白并不会构成影响T-Ag运动的一个障碍，后者能够将该“六聚物”破开，绕过这种蛋白路障。%Replicative DNA helicases generally unwind DNA as a single hexamer that encircles and translocates along one strand of the duplex while excluding the complementary strand (known as steric exclusion). By contrast, large T antigen, the replicative DNA helicase of the simian virus 40 (SV40), is reported to function as a pair of stacked hexamers that pumps double-stranded DNA through its central channel while laterally extruding single-stranded DNA. Here we use single-molecule and ensemble assays to show that large T antigen assembled on the SV40 origin unwinds DNA efficiently as a single hexamer that translocates on single-stranded DNA in the 3'-to-5' direction. Unexpectedly, large T antigen unwinds DNA past a DNA-protein crosslink on the translocation strand, suggesting that the large T antigen ring can open to bypass bulky adducts. Together, our data underscore the profound conservation among replicative helicase mechanisms, and reveal a new level of plasticity in the interactions of replicative helicases with DNA damage.