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The ATG12~ATG5 conjugate promotes the transfer of the ubiquitin-like protein LC3 to phosphatidylethanolamine (PE), a modification required for autophagosome formation. Structural and biochemical analyses reveal the determinants for ATG12~ATG5 binding to ATG16 and the E3 ligase ATG3, and indicate how the conjugate stimulates PE–LC3 formation. The autophagy factor ATG12~ATG5 conjugate exhibits E3 ligase–like activity which facilitates the lipidation of members of the LC3 family. The crystal structure of the human ATG12~ATG5 conjugate bound to the N-terminal region of ATG16L1, the factor that recruits the conjugate to autophagosomal membranes, reveals an integrated architecture in which ATG12 docks onto ATG5 through conserved residues. ATG12 and ATG5 are oriented such that other conserved residues on each molecule, including the conjugation junction, form a continuous surface patch. Mutagenesis data support the importance of both the interface between ATG12 and ATG5 and the continuous patch for E3 activity. The ATG12~ATG5 conjugate interacts with the E2 enzyme ATG3 with high affinity through another surface location that is exclusive to ATG12, suggesting a different role of the continuous patch in E3 activity. These findings provide a foundation for understanding the mechanism of LC3 lipidation.

The autophagic ubiquitin-like protein (ublp) autophagy-related (ATG)12 is a component of the ATG12∼ATG5–ATG16L1 E3 complex that promotes lipid conjugation of members of the LC3 ublp family. A role of ATG12 in the E3 complex is to recruit the E2 enzyme ATG3. Here we report the identification of the ATG12 binding sequence in the flexible region of human ATG3 and the crystal structure of the minimal E3 complexed with the identified binding fragment of ATG3. The structure shows that 13 residues of the ATG3 fragment form a short β-strand followed by an α-helix on a surface area that is exclusive to ATG12. Mutational analyses of ATG3 confirm that four residues whose side chains make contacts with ATG12 are important for E3 interaction as well as LC3 lipidation. Conservation of these four critical residues is high in metazoan organisms and plants but lower in fungi. A structural comparison reveals that the ATG3 binding surface on ATG12 contains a hydrophobic pocket corresponding to the binding pocket of LC3 that accommodates the leucine of the LC3-interacting region motif. These findings establish the mechanism of ATG3 recruitment by ATG12 in higher eukaryotes and place ATG12 among the members of signaling ublps that bind liner sequences.

Identification of broadly neutralizing antibodies against influenza A viruses has raised hopes for the development of monoclonal antibody—based immunotherapy and \"universal\" vaccines for influenza. However, a substantial part of the annual flu burden is caused by two cocirculating, antigenically distinct lineages of influenza B viruses. Here, we report human monoclonal antibodies, CR8033, CR8071, and CR9114, that protect mice against lethal challenge from both lineages. Antibodies CR8033 and CR8071 recognize distinct conserved epitopes in the head region of the influenza B hemagglutinin (HA), whereas CR9114 binds a conserved epitope in the HA stem and protects against lethal challenge with influenza A and B viruses. These antibodies may inform on development of monoclonal antibody—based treatments and a universal flu vaccine for all influenza A and B viruses.

Members of the Wiskott–Aldrich syndrome protein (WASP) family control cytoskeletal dynamics by promoting actin filament nucleation with the Arp2/3 complex. The WASP relative WAVE regulates lamellipodia formation within a 400-kilodalton, hetero-pentameric WAVE regulatory complex (WRC). The WRC is inactive towards the Arp2/3 complex, but can be stimulated by the Rac GTPase, kinases and phosphatidylinositols. Here we report the 2.3-ångstrom crystal structure of the WRC and complementary mechanistic analyses. The structure shows that the activity-bearing VCA motif of WAVE is sequestered by a combination of intramolecular and intermolecular contacts within the WRC. Rac and kinases appear to destabilize a WRC element that is necessary for VCA sequestration, suggesting the way in which these signals stimulate WRC activity towards the Arp2/3 complex. The spatial proximity of the Rac binding site and the large basic surface of the WRC suggests how the GTPase and phospholipids could cooperatively recruit the complex to membranes. WAVE control of actin polymerization The WAVE protein is a central regulator of actin dynamics during cell motility. WAVE is a member of the Wiskott–Aldrich syndrome protein (WASP) family, which promotes the actin-filament-nucleating activity of the Arp2/3 complex. In cells, WAVE is constitutively incorporated into the 350-kilodalton WAVE regulatory complex (WRC); it is normally present in an inactive state and can be activated by a number of inputs including the RacGTPase. Here, Chen et al . present the structure and mechanistic analysis of the WRC. The combined data reveal how the WAVE protein is inhibited within the WRC complex and provide mechanisms for WRC activation at the plasma membrane. In cells, WAVE protein, a central regulator of actin dynamics during cell motility, is constitutively incorporated into WAVE regulatory complex (WRC), is normally present in an inactive state and can be activated by a number of inputs. These authors present the structure and mechanistic analysis of WRC. The combined data reveal how the WAVE protein is inhibited within the WRC complex and provide mechanisms for WRC activation at the plasma membrane.

We recently showed that the Wiskott-Aldrich syndrome protein (WASP) family member, WASH, localizes to endosomal subdomains and regulates endocytic vesicle scission in an Arp2/3-dependent manner. Mechanisms regulating WASH activity are unknown. Here we show that WASH functions in cells within a 500 kDa core complex containing Strumpellin, FAM21, KIAA1033 (SWIP), and CCDC53. Although recombinant WASH is constitutively active toward the Arp2/3 complex, the reconstituted core assembly is inhibited, suggesting that it functions in cells to regulate actin dynamics through WASH. FAM21 interacts directly with CAPZ and inhibits its actin-capping activity. Four of the five core components show distant (approximately 15% amino acid sequence identify) but significant structural homology to components of a complex that negatively regulates the WASP family member, WAVE. Moreover, biochemical and electron microscopic analyses show that the WASH and WAVE complexes are structurally similar. Thus, these two distantly related WASP family members are controlled by analogous structurally related mechanisms. Strumpellin is mutated in the human disease hereditary spastic paraplegia, and its link to WASH suggests that misregulation of actin dynamics on endosomes may play a role in this disorder.

In this study, we aimed to encapsulate the sizable double-stranded DNA (dsDNA, 3.9 kbp) into a small-sized infectious hypodermal and hematopoietic necrosis virus-like particle (IHHNV-VLP; T = 1) and compared the changes in capsid structure between dsDNA-filled VLP and empty VLP. Based on our encapsulation protocol, IHHNV-VLP was able to load dsDNA at an efficiency of 30–40% (w/w) into its cavity. Structural analysis revealed two subclasses of IHHNV-VLP, so-called empty and dsDNA-filled VLPs. The three-dimensional (3D) structure of the empty VLP produced in E. coli was similar to that of the empty IHHNV-VLP produced in Sf9 insect cells. The size of the dsDNA-filled VLP was slightly bigger (50 Å) than its empty VLP counterpart; however, the capsid structure was drastically altered. The capsid was about 1.5-fold thicker due to the thickening of the capsid interior, presumably from DNA–capsid interaction evident from capsid protrusions or nodules on the interior surface. In addition, the morphological changes of the capsid exterior were particularly observed in the vicinity of the five-fold axes, where the counter-clockwise twisting of the “tripod” structure at the vertex of the five-fold channel was evident, resulting in a widening of the channel’s opening. Whether these capsid changes are similar to virion capsid maturation in the host cells remains to be investigated. Nevertheless, the ability of IHHNV-VLP to encapsulate the sizable dsDNA has opened up the opportunity to package a dsDNA vector that can insert exogenous genes and target susceptible shrimp cells in order to halt viral infection.

In this Resource study, the authors used high-resolution mass spectrometry to elucidate the precise proteomic complement of the inner ear hair bundle. Many of the proteins that are enriched in the hair bundles are encoded by known deafness-associated genes. Hair bundles of the inner ear have a specialized structure and protein composition that underlies their sensitivity to mechanical stimulation. Using mass spectrometry, we identified and quantified >1,100 proteins, present from a few to 400,000 copies per stereocilium, from purified chick bundles; 336 of these were significantly enriched in bundles. Bundle proteins that we detected have been shown to regulate cytoskeleton structure and dynamics, energy metabolism, phospholipid synthesis and cell signaling. Three-dimensional imaging using electron tomography allowed us to count the number of actin-actin cross-linkers and actin-membrane connectors; these values compared well to those obtained from mass spectrometry. Network analysis revealed several hub proteins, including RDX (radixin) and SLC9A3R2 (NHERF2), which interact with many bundle proteins and may perform functions essential for bundle structure and function. The quantitative mass spectrometry of bundle proteins reported here establishes a framework for future characterization of dynamic processes that shape bundle structure and function.