Myosins are molecular motor proteins that harness the chemical energy stored in ATP to produce directed force along actin filaments. Complex communication pathways link the catalytic nucleotide-binding region, the structures responsible for force amplification and the actin-binding domain of myosin. We have crystallized the nucleotide-free motor domain of myosin II in a new conformation in which switch I and switch II, conserved loop structures involved in nucleotide binding, have moved away from the nucleotide-binding pocket. These movements are linked to rearrangements of the actin-binding region, which illuminate a previously unobserved communication pathway between the nucleotide-binding pocket and the actin-binding region, explain the reciprocal relationship between actin and nucleotide affinity and suggest a new mechanism for product release in myosin family motors.

The apoptotic protease-activating factor 1 (Apaf-1) relays the death signal in the mitochondrial pathway of apoptosis. Apaf-1 oligomerizes on binding of mitochondrially released cytochrome c into the heptameric apoptosome complex to ignite the downstream cascade of caspases. Here, we present the 3.0 Å crystal structure of full-length murine Apaf-1 in the absence of cytochrome c. The structure shows how the mammalian death switch is kept in its "off" position. By comparing the off state with a recent cryo-electron microscopy derived model of Apaf-1 in its apoptosomal conformation, we depict the molecular events that transform Apaf-1 from autoinhibited monomer to a building block of the caspase-activating apoptosome. Moreover, we have solved the crystal structure of the R265S mutant of full-length murine Apaf-1 in the absence of cytochrome c to 3.55 Å resolution and we show that proper function of Apaf-1 relies on R265 in the vicinity of the bound nucleotide.

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...