SARS-CoV-2 Infection Depends on Cellular Heparan Sulfate and ACE2.

We show that SARS-CoV-2 spike protein interacts with both cellular heparan sulfate and angiotensin-converting enzyme 2 (ACE2) through its receptor-binding domain (RBD). Docking studies suggest a heparin/heparan sulfate-binding site adjacent to the ACE2-binding site. Both ACE2 and heparin can bind independently to spike protein in vitro, and a ternary complex can be generated using heparin as a scaffold. Electron micrographs of spike protein suggests that heparin enhances the open conformation of the RBD that binds ACE2. On cells, spike protein binding depends on both heparan sulfate and ACE2. Unfractionated heparin, non-anticoagulant heparin, heparin lyases, and lung heparan sulfate potently block spike protein binding and/or infection by pseudotyped virus and authentic SARS-CoV-2 virus. We suggest a model in which viral attachment and infection involves heparan sulfate-dependent enhancement of binding to ACE2. Manipulation of heparan sulfate or inhibition of viral adhesion by exogenous heparin presents new therapeutic opportunities.

A Redox Role for the [4Fe4S] Cluster of Yeast DNA Polymerase δ.

A [4Fe4S]2+ cluster in the C-terminal domain of the catalytic subunit of the eukaryotic B-family DNA polymerases is essential for the formation of active multi-subunit complexes. Here we use a combination of electrochemical and biochemical methods to assess the redox activity of the [4Fe4S]2+ cluster in Saccharomyces cerevisiae polymerase (Pol) δ, the lagging strand DNA polymerase. We find that Pol δ bound to DNA is indeed redox-active at physiological potentials, generating a DNA-mediated signal electrochemically with a midpoint potential of 113 ± 5 mV versus NHE. Moreover, biochemical assays following electrochemical oxidation of Pol δ reveal a significant slowing of DNA synthesis that can be fully reversed by reduction of the oxidized form. A similar result is apparent with photooxidation using a DNA-tethered anthraquinone. These results demonstrate that the [4Fe4S] cluster in Pol δ can act as a redox switch for activity, and we propose that this switch can provide a rapid and reversible way to respond to replication stress.

Sulfur K-Edge XAS Studies of the Effect of DNA Binding on the [Fe4S4] Site in EndoIII and MutY.

S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe-S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron-thiolate and iron-sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron-sulfur protein (HiPIP). A limited decrease in covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron-sulfur bonds. In EndoIII, this change in covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior.

Electrochemistry of the [4Fe4S] Cluster in Base Excision Repair Proteins: Tuning the Redox Potential with DNA.

Escherichia coli endonuclease III (EndoIII) and MutY are DNA glycosylases that contain [4Fe4S] clusters and that serve to maintain the integrity of the genome after oxidative stress. Electrochemical studies on highly oriented pyrolytic graphite (HOPG) revealed that DNA binding by EndoIII leads to a large negative shift in the midpoint potential of the cluster, consistent with stabilization of the oxidized [4Fe4S]3+ form. However, the smooth, hydrophobic HOPG surface is nonideal for working with proteins in the absence of DNA. In this work, we use thin film voltammetry on a pyrolytic graphite edge electrode to overcome these limitations. Improved adsorption leads to substantial signals for both EndoIII and MutY in the absence of DNA, and a large negative potential shift is retained with DNA present. In contrast, the EndoIII mutants E200K, Y205H, and K208E, which provide electrostatic perturbations in the vicinity of the cluster, all show DNA-free potentials within error of wild type; similarly, the presence of negatively charged poly-l-glutamate does not lead to a significant potential shift. Overall, binding to the DNA polyanion is the dominant effect in tuning the redox potential of the [4Fe4S] cluster, helping to explain why all DNA-binding proteins with [4Fe4S] clusters studied to date have similar DNA-bound potentials.

A human MUTYH variant linking colonic polyposis to redox degradation of the [4Fe4S]2+ cluster.

The human DNA repair enzyme MUTYH excises mispaired adenine residues in oxidized DNA. Homozygous MUTYH mutations underlie the autosomal, recessive cancer syndrome MUTYH-associated polyposis. We report a MUTYH variant, p.C306W (c.918C>G), with a tryptophan residue in place of native cysteine, that ligates the [4Fe4S] cluster in a patient with colonic polyposis and family history of early age colon cancer. In bacterial MutY, the [4Fe4S] cluster is redox active, allowing rapid localization to target lesions by long-range, DNA-mediated signalling. In the current study, using DNA electrochemistry, we determine that wild-type MUTYH is similarly redox-active, but MUTYH C306W undergoes rapid oxidative degradation of its cluster to [3Fe4S]+, with loss of redox signalling. In MUTYH C306W, oxidative cluster degradation leads to decreased DNA binding and enzyme function. This study confirms redox activity in eukaryotic DNA repair proteins and establishes MUTYH C306W as a pathogenic variant, highlighting the essential role of redox signalling by the [4Fe4S] cluster.

Electrical Probes of DNA-Binding Proteins.

A DNA electrochemistry platform has been developed to probe proteins bound to DNA electrically. Here gold electrodes are modified with thiol-modified DNA, and DNA charge transport chemistry is used to probe DNA binding and enzymatic reaction both with redox-silent and redox-active proteins. For redox-active proteins, the electrochemistry permits the determination of redox potentials in the DNA-bound form, where comparisons to DNA-free potentials can be made using graphite electrodes without DNA modification. Importantly, electrochemistry on the DNA-modified electrodes facilitates reaction under aqueous, physiological conditions with a sensitive electrical measurement of binding and activity.