Single-molecule analysis of DNA-binding proteins from nuclear extracts (SMADNE).

Single-molecule characterization of protein-DNA dynamics provides unprecedented mechanistic details about numerous nuclear processes. Here, we describe a new method that rapidly generates single-molecule information with fluorescently tagged proteins isolated from nuclear extracts of human cells. We demonstrated the wide applicability of this novel technique on undamaged DNA and three forms of DNA damage using seven native DNA repair proteins and two structural variants, including: poly(ADP-ribose) polymerase (PARP1), heterodimeric ultraviolet-damaged DNA-binding protein (UV-DDB), and 8-oxoguanine glycosylase 1 (OGG1). We found that PARP1 binding to DNA nicks is altered by tension, and that UV-DDB did not act as an obligate heterodimer of DDB1 and DDB2 on UV-irradiated DNA. UV-DDB bound to UV photoproducts with an average lifetime of 39 seconds (corrected for photobleaching, τc), whereas binding lifetimes to 8-oxoG adducts were < 1 second. Catalytically inactive OGG1 variant K249Q bound oxidative damage 23-fold longer than WT OGG1, at 47 and 2.0 s, respectively. By measuring three fluorescent colors simultaneously, we also characterized the assembly and disassembly kinetics of UV-DDB and OGG1 complexes on DNA. Hence, the SMADNE technique represents a novel, scalable, and universal method to obtain single-molecule mechanistic insights into key protein-DNA interactions in an environment containing physiologically-relevant nuclear proteins.

Topological analysis of a bacterial DedA protein associated with alkaline tolerance and antimicrobial resistance.

Maintaining membrane integrity is of paramount importance to the survival of bacteria as the membrane is the site of multiple crucial cellular processes including energy generation, nutrient uptake and antimicrobial efflux. The DedA family of integral membrane proteins are widespread in bacteria and are associated with maintaining the integrity of the membrane. In addition, DedA proteins have been linked to resistance to multiple classes of antimicrobials in various microorganisms. Therefore, the DedA family are attractive targets for the development of new antibiotics. Despite DedA family members playing a key physiological role in many bacteria, their structure, function and physiological role remain unclear. To help illuminate the structure of the bacterial DedA proteins, we performed substituted cysteine accessibility method (SCAM) analysis on the most comprehensively characterized bacterial DedA protein, YqjA from Escherichia coli. By probing the accessibility of 15 cysteine residues across the length of YqjA using thiol reactive reagents, we mapped the topology of the protein. Using these data, we experimentally validated a structural model of YqjA generated using evolutionary covariance, which consists of an α-helical bundle with two re-entrant hairpin loops reminiscent of several secondary active transporters. In addition, our cysteine accessibility data suggest that YqjA forms an oligomer wherein the protomers are arranged in a parallel fashion. This experimentally verified model of YqjA lays the foundation for future work in understanding the function and mechanism of this interesting and important family.