ssDNA diffuses along replication protein A via a reptation mechanism

Abstract Replication protein A (RPA) plays a critical role in all eukaryotic DNA processing involving single-stranded DNA (ssDNA). Contrary to the notion that RPA provides solely inert protection to transiently formed ssDNA, the RPA–ssDNA complex acts as a dynamic DNA processing unit. Here, we studied the diffusion of RPA along 60 nt ssDNA using a coarse-grained model in which the ssDNA–RPA interface was modeled by both aromatic and electrostatic interactions. Our study provides direct evidence of bulge formation during the diffusion of ssDNA along RPA. Bulges can form at a few sites along the interface and store 1–7 nt of ssDNA whose release, upon bulge dissolution, leads to propagation of ssDNA diffusion. These findings thus support the reptation mechanism, which involves bulge formation linked to the aromatic interactions, whose short range nature reduces cooperativity in ssDNA diffusion. Greater cooperativity and a larger diffusion coefficient for ssDNA diffusion along RPA are observed for RPA variants with weaker aromatic interactions and for interfaces homogenously stabilized by electrostatic interactions. ssDNA propagation in the latter instance is characterized by lower probabilities of bulge formation; thus, it may fit the sliding-without-bulge model better than the reptation model. Thus, the reptation mechanism allows ssDNA mobility despite the extensive and high affinity interface of RPA with ssDNA. The short-range aromatic interactions support bulge formation while the long-range electrostatic interactions support the release of the stored excess ssDNA in the bulge and thus the overall diffusion.

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Mechanism of Dynamic Binding of Replication Protein A to ssDNA

10.1101/2020.03.16.994681 ◽

2020 ◽

Author(s):

Anupam Mondal ◽

Arnab Bhattacherjee

Keyword(s):

Dna Repair ◽

Dynamic Equilibrium ◽

Diffusion Mechanism ◽

Protein A ◽

Principal Component ◽

Replication Protein A ◽

Replication Protein ◽

Coarse Grained ◽

Dna Processing

AbstractReplication protein A (RPA) serves as hub protein inside eukaryotic cells, where it coordinates crucial DNA metabolic processes and activates the DNA-damage response system. A characteristic feature of its action is to associate with ssDNA intermediates before handing over them to downstream proteins. The length of ssDNA intermediates differs for different pathways. This means RPA must have mechanisms for selective processing of ssDNA intermediates based on their length, the knowledge of which is fundamental to elucidate when and how DNA repair and replication processes are symphonized. By employing extensive molecular simulations, we investigated the mechanism of binding of RPA to ssDNA of different lengths. We show that the binding involves dynamic equilibrium with a stable intermediate, the population of which increases with the length of ssDNA. The vital underlying factors are decoded through collective variable principal component analysis. It suggests a differently orchestrated set of interactions that define the action of RPA based on the sizes of ssDNA intermediates. We further estimated the association kinetics and probed the diffusion mechanism of RPA to ssDNA. RPA diffuses on short ssDNA through progressive ‘bulge’ formation. With long ssDNA, we observed a conformational change in ssDNA coupled with its binding to RPA in a cooperative fashion. Our analysis explains how the ‘short-lived,’ long ssDNA intermediates are processed quickly in vivo. The study thus reveals the molecular basis of several recent experimental observations related to RPA binding to ssDNA and provides novel insights into the RPA functioning in DNA repair and replication.Significance StatementDespite ssDNA be the common intermediate to all pathways involving RPA, how does the latter function differently in the DNA processing events such as DNA repair, replication, and recombination just based on the length of ssDNA intermediates remains unknown. The major hindrance is the difficulty in capturing the transient interactions between the molecules. Even attempts to crystallize RPA complexes with 32nt and 62nt ssDNA have yielded a resolved structure of only 25nt ssDNA wrapped with RPA. Here, we used a state-of-the-art coarse-grained protein-ssDNA model to unravel the detailed mechanism of binding of RPA to ssDNA. Our study illustrates the molecular origin of variations in RPA action during various DNA processing events depending on the length of ssDNA intermediates.

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