Significance The Smc5/6 complex plays multiple roles in DNA replication and repair. Its genome-protecting functions rely on its interaction with DNA; however, how this complex engages DNA is poorly understood. We report on a cryogenic electron microscopy structure of DNA-bound budding yeast Smc5/6 complex, revealing that its subunits form a clamp to encircle a double-helical DNA. We define the multi-subunit interactions forming the DNA clamp and the DNA binding sites distributed among subunits. We identify subunit transformations upon DNA capture and functional effects conferred by its multiple DNA contact sites. Our findings, in conjunction with studies on other structural maintenance of chromosomes (SMC) complexes, suggest a common SMC DNA-clamp mechanism with individual complex specific features that enable diverse genome organization and protection functions by SMC family complexes.

Structural maintenance of chromosomes (SMC) complexes are critical chromatin modulators. In eukaryotes, the cohesin and condensin SMC complexes organize chromatin, while the Smc5/6 complex directly regulates DNA replication and repair. The molecular basis for the distinct functions of Smc5/6 is poorly understood. Here, we report an integrative structural study of the budding yeast Smc5/6 holo-complex using electron microscopy, cross-linking mass spectrometry, and computational modeling. We show that the Smc5/6 complex possesses several unique features, while sharing some architectural characteristics with other SMC complexes. In contrast to arm-folded structures of cohesin and condensin, Smc5 and Smc6 arm regions do not fold back on themselves. Instead, these long filamentous regions interact with subunits uniquely acquired by the Smc5/6 complex, namely the Nse2 SUMO ligase and the Nse5/Nse6 subcomplex, with the latter also serving as a linchpin connecting distal parts of the complex. Our 3.0-Å resolution cryoelectron microscopy structure of the Nse5/Nse6 core further reveals a clasped-hand topology and a dimeric interface important for cell growth. Finally, we provide evidence that Nse5/Nse6 uses its SUMO-binding motifs to contribute to Nse2-mediated sumoylation. Collectively, our integrative study identifies distinct structural features of the Smc5/6 complex and functional cooperation among its coevolved unique subunits.

Biodegradable nucleobase-grafted amphiphilic copolymer, the methoxyl poly (ethylene glycol)-b-poly (L-lactide-co-2-methyl-2(3-(2,3-dihydroxylpropylthio) propyloxycarbonyl)-propylene carbonate/1-carboxymethylthymine) (mPEG-b- P(LA-co-MPT)), was synthesized. (1)H NMR titration and FT-IR spectroscopy indicated that the hydrogen-bonding could be formed between mPEG-b-P(LA-co-MPT) and 9-hexadecyladenine (A-C16). The hydrophobic microenvironment of the amphiphilic copolymer can protect the complementary multiple hydrogen bonds between mPEG-b-P(LA-co-MPT) and A-C16 from water effectively. The addition of A-C16 not only lowered the critical aggregation concentration (CAC) of mPEG-b-P(LA-co-MPT)/A-C16 nanoparticles (NPs) in aqueous solution but also induced different morphologies, which can be observed by transmission electron microscopy (TEM). Meanwhile, dynamic light scattering (DLS) and turbidometry was utilized to evaluate the effect of temperature and pH change on the stability of mPEG-b-P(LA-co-MPT)/A-C16 NPs. Cytotoxicity evaluation showed good biocompatibility of the mPEG-b-P(LA-co-MPT)/A-C16 NPs. The in vitro drug release profile showed that with the increase of A-C16 content, the doxorubiucin (DOX) release at pH 7.4 decreased, while the faster release rate was observed with the addition of A-C16 with a pH of 5.0. Importantly, DOX-loaded NPs exerted comparable cytotoxicity against MDA-MB-231 cells. This work provided a new method to stabilize NP structure using hydrogen-bonds and would have the potential to be applied in controlled drug delivery.