The coronavirus disease 2019 pandemic caused by severe acute respiratory syndromecoronavirus 2 (SARS-CoV-2) is a global health emergency. An attractive drug target among coronaviruses is the main protease (M pro , also called 3CL pro ) because of its essential role in processing the polyproteins that are translated from the viral RNA. We report the x-ray structures of the unliganded SARS-CoV-2 M pro and its complex with an a-ketoamide inhibitor. This was derived from a previously designed inhibitor but with the P3-P2 amide bond incorporated into a pyridone ring to enhance the half-life of the compound in plasma. On the basis of the unliganded structure, we developed the lead compound into a potent inhibitor of the SARS-CoV-2 M pro . The pharmacokinetic characterization of the optimized inhibitor reveals a pronounced lung tropism and suitability for administration by the inhalative route.

Molecular mechanism of Ena/VASP-mediated actin-filament elongationEna/VASP proteins have important functions in actin-dependent processes. A model for the actin elongation activity of Ena/VASP based on the affinity and saturation state of WH2-domain-mediated actin monomer binding is presented.

We solved the crystal structure of the homotetrameric single-stranded DNA binding (SSB) protein from human mitochondria at a resolution of 2.4 A. The tetramer is formed by two dimers interacting head-to-head and shows D2 symmetry. Sequence-related tetrameric SSB proteins occur in prokaryotes and eukaryotic mitochondria; this is the first report of an atomic resolution structure of this type of protein. Using biochemical data and analysis of sequence homologies, we were able to correlate the functional properties with structure. We propose that ssDNA wraps around the tetrameric HsmtSSB protein through electropositive channels guided by flexible loops.

The mechano-chemical protein dynamin is the prototype of the dynamin superfamily of large GTPases, which shape and remodel membranes in diverse cellular processes 1 .Dynamin forms predominantly tetramers in the cytosol, which oligomerize at the neck of clathrin-coated vesicles to mediate constriction and subsequent scission of the membrane 1 . Previous studies have described the architecture of dynamin dimers 2,3 , but the molecular determinants for dynamin assembly and its regulation have remained unclear. Here, we present the crystal structure of the nucleotide-free dynamin tetramer.Combining structural data with mutational studies, oligomerization measurements and molecular dynamics simulations, we suggest a mechanism of how oligomerization of dynamin is linked to the release of intramolecular auto-inhibitory interactions. We elucidate how mutations that interfere with tetramer formation and auto-inhibition can lead to the congenital diseases Charcot-Marie-Tooth neuropathy (CMT) 4 and centronuclear myopathy (CNM) 5 , respectively. Strikingly, the bent shape of the tetramer explains how dynamin assembles into a right-handed helical oligomer of defined diameter, which has direct implications for its function in membrane constriction.The three highly conserved vertebrate isoforms of dynamin contain five distinct domains (Extended Data Fig. 1a): an N-terminal GTPase (G) domain mediating nucleotide binding and hydrolysis, a bundle signaling element (BSE), a stalk, a pleckstrin homology (PH) domain involved in lipid binding, and a proline rich domain (PRD) mediating interaction with BAR-and SH3-domain containing scaffolding proteins 6 . To exert its function in clathrinmediated endocytosis (CME), dynamin assembles via the stalks into a helical array surrounding the necks of invaginating clathrin-coated pits (CCP) 7,8 . Dimerization of GTPbound G domains from neighboring helical rungs induces GTP hydrolysis 9 . The ensuing conformational changes are thought to be transmitted from the G domain via the BSE to the stalk resulting in a sliding motion of adjacent helix rungs, concomitant helix constriction 10 , and eventually membrane scission. The inherent tendency to form large assemblies at high protein concentrations has hampered crystallization of dynamin in the past. The use of non-3 oligomerizing mutants led to crystal structures of dynamin 1 2,3 . However, the postulated higher-order assembly interface was not resolved in these structures leaving the oligomerization mechanism unaddressed.We reasoned that an alternative assembly-affecting mutation, such as K361S in dynamin 3 11 , may disturb the oligomerization interface to a lesser extent than the previously used mutants. We obtained crystals of nucleotide-free dynamin 3-K361S lacking the PRD (dynamin 3(∆PRD)-K361S) that diffracted to 3.7 Å (Methods, Extended Data Fig. 1, Extended Data Table 1). Indeed, the asymmetric unit of the crystal lattice contained a dynamin tetramer that did not form the filamentous superstructures seen for dynamin 1 2,3 .The dynamin tetr...