Direct inhibition of smooth muscle myosin (SMM) is a potential means to treat hypercontractile smooth muscle diseases. The selective inhibitor CK-2018571 prevents strong binding to actin and promotes muscle relaxation in vitro and in vivo. The crystal structure of the SMM/drug complex reveals that CK-2018571 binds to a novel allosteric pocket that opens up during the "recovery stroke" transition necessary to reprime the motor. Trapped in an intermediate of this fast transition, SMM is inhibited with high selectivity compared with skeletal muscle myosin (IC 50 = 9 nM and 11,300 nM, respectively), although all of the binding site residues are identical in these motors. This structure provides a starting point from which to design highly specific myosin modulators to treat several human diseases. It further illustrates the potential of targeting transition intermediates of molecular machines to develop exquisitely selective pharmacological agents.M yosins comprise a family of ATP-dependent motor proteins capable of producing directed force via interaction with their track, the F-actin filament. Force production by these motors powers numerous cellular processes such as muscle contraction, intracellular transport, and cell migration and division (1). Several myosins have also been linked to genetic disorders where either gain or loss of motor function can lead to disease. These motor proteins represent promising targets for the development of drugs modulating force production in cells, tissues, and muscle (2-4). Here we report a selective, smallmolecule inhibitor of smooth muscle myosin (SMM) able to induce muscle relaxation. This mechanism of action has potential relevance for many diseases where smooth muscle contractility is central to the pathophysiology, such as asthma (5, 6) and chronic obstructive pulmonary disease (7).Smooth muscle contractility can be activated through different pathways. Existing airway smooth muscle relaxants, such as β-adrenergic agonists and muscarinic antagonists, ultimately inhibit the activity of SMM. However, they do so via specific upstream signaling pathways. Direct inhibition of SMM contractility has the advantage of relaxing contracted smooth muscle regardless of the molecular stimulus driving it. Moreover, application of SMM inhibitors to the airway provides a means of selectively modulating contractility of these tissues by delivering a high local concentration of drug. We thus set about identifying selective inhibitors of SMM that can effectively relax muscle in vivo, leading to the discovery of a highly selective, small-molecule inhibitor, CK-2018571 (CK-571).The detailed inhibitory mechanism of CK-571 was elucidated by a combination of in vitro characterization of the step in which the drug traps the motor and determination of the high-resolution structure of SMM cocrystallized with CK-571. The drug targets an intermediate state that occurs during the recovery stroke, the large conformational rearrangement that enables repriming of the motor. Blocking this critical transiti...

Bacillithiol is a glucosamine‐derived antioxidant found in several pathogenic Gram‐positive bacteria. The compound is involved in maintaining the appropriate redox state within the cell as well as detoxifying foreign agents like the antibiotic fosfomycin. Bacillithiol is produced via the action of three enzymes, including BshA, a retaining GT‐B glycosyltransferase that utilizes UDP‐N‐acetylglucosamine and l‐malate to produce N‐acetylglucosaminyl‐malate. Recent studies suggest that retaining GT‐B glycosyltransferases like BshA utilize a substrate‐assisted mechanism that goes through an SNi‐like transition state. In a previous study, we relied on X‐ray crystallography as well as computational simulations to hypothesize the manner in which substrates would bind the enzyme, but several questions about substrate binding and the role of one of the amino acid residues persisted. Another study demonstrated that BshA might be subject to feedback inhibition by bacillithiol, but this phenomenon was not analyzed further to determine the exact mechanism of inhibition. Here we present X‐ray crystallographic structures and steady‐state kinetics results that help elucidate both of these issues. Our ligand‐bound crystal structures demonstrate that the active site provides an appropriate steric and geometric arrangement of ligands to facilitate the substrate‐assisted mechanism. Finally, we show that bacillithiol is competitive for UDP‐N‐acetylglucosamine with a Ki value near 120–130 μM and likely binds within the BshA active site, suggesting that bacillithiol modulates BshA activity via feedback inhibition. The work presented here furthers our understanding of bacillithiol metabolism and can aid in the development of inhibitors to counteract resistance to antibiotics such as fosfomycin.

Bacillithiol is a low-molecular weight thiol produced by many gram-positive organisms, including Staphylococcus aureus and Bacillus anthracis. It is the major thiol responsible for maintaining redox homeostasis and cellular detoxification, including inactivation of the antibiotic fosfomycin. The metal-dependent bacillithiol transferase BstA is likely involved in these sorts of detoxification processes, but the exact substrates and enzyme mechanism have not been identified. Here we report the 1.34 Å resolution X-ray crystallographic structure of BstA from S. aureus. Our structure confirms that BstA belongs to the YfiT-like metal-dependent hydrolase superfamily. Like YfiT, our structure contains nickel within its active site, but our functional data suggest that BstA utilizes zinc for activity. Although BstA and YfiT both contain a core four helix bundle and coordinate their metal ions in the same fashion, significant differences between the protein structures are described here.

Bacillithiol (BSH) is the major low molecular weight thiol in many gram-positive organisms. This compound is involved in the detoxification of xenobiotic compounds, the maintenance of redox homeostasis, and is the preferred cosubstrate for the fosfomycin resistance enzyme FosB. Organisms with knocked-out BSH biosynthesis genes have demonstrated an increased sensitivity to fosfomycin, making BSH production a logical target for inhibition. In the first committed step of the BSH biosynthesis pathway, the enzyme BshA catalyzes the formation of N-acetylglucosaminylmalate (GlcNAc-Mal) via an activated sugar donor and l-malate acceptor. BshA is a GT-B retaining glycosyltransferase and likely utilizes an SNi-like retaining mechanism. To further characterize BshA and other GT-B retaining glycosyltransferases, we embarked on a structural and functional analysis of BshA. Our lab has determined several X-ray crystallographic structures of wild-type BshA from Bacillus subtilis with products bound. In addition, we have determined an initial structure of the S17A mutant of BshA from Bacillus subtilis that contains the substrate UDP-N-acetylglucosamine bound within the active site. These structures provide compelling evidence for the proposed SNi-like mechanism. We also report progress on the structural and functional analysis of the BshA enzyme from Staphylococcus saprophyticus. Together, these analyses further our understanding of BshA and other GT-B retaining glycosyltransferases, which may be used in the future to model BshA inhibitors and increase the effectiveness of fosfomycin antibiotics.