Overall architecture of the giant E3 ligase HUWE1_N.

a, Domain architecture of HUWE1_N. ARM repeats 1–34 are numbered, with the four insertions indicated. The positions of human HUWE1 insertions, absent in HUWE1_N, are shown in brackets. b, Crystal structure of HUWE1_N. c, Crystal structure of HUWE1_N shown in cartoon representation from four different views, using the same color coding as in a (catalytic Cys in red). A schematic cartoon illustrates the snake-like organization of the E3 ligase. d, Negative-stain EM analysis of CeHUWE1. The obtained EM density is shown from two viewpoints, with approximate dimensions indicated. e, Organization and increasing complexity of HUWE1.

Tim Clausen Research Group

Significance

HUWE1 is a universal quality-control E3 ligase that marks diverse client proteins for proteasomal degradation. Although the giant HECT enzyme is an essential component of the ubiquitin–proteasome system closely linked with severe human diseases, its molecular mechanism is little understood. Here, we present the crystal structure of NematocidaHUWE1, revealing how a single E3 enzyme has specificity for a multitude of unrelated substrates. The protein adopts a remarkable snake-like structure, where the C-terminal HECT domain heads an extended alpha-solenoid body that coils in on itself and houses various protein–protein interaction modules. Our integrative structural analysis shows that this ring structure is highly dynamic, enabling the flexible HECT domain to reach protein targets presented by the various acceptor sites. Together, our data demonstrate how HUWE1 is regulated by its unique structure, adapting a promiscuous E3 ligase to selectively target unassembled orphan proteins.

Grabarczyk, D.B., Petrova,O.A., Deszcz, L., Kurzbauer, R., Murphy, P., Ahel, J., Vogel, A., Gogova, R., Faas, V., Kordic, D., Schleiffer, A., Meinhart, A:, Imre, R., Lehner, A., Neuhold, J., Bader, G., Stolt-Bergner, P., Böttcher, J., Wolkerstorfer, B., Fischer, G.,  Grishkovskaya, I., Haselbach, D., Kessler,D., and Clausen T.: HUWE1 employs a giant substrate-binding ring to feed and regulate its HECT E3 domain, Nat. Chem. Biol. (2021) https://doi.org/10.1038/s41589-021-00831-5

Structure of virion of rhinovirus 14 contains resolved density corre- sponding to octanucleotides from its RNA genome. (A) Surface representa- tion of cryo-EM of reconstruction of virion of rhinovirus 14 with front half of the particle removed to show internal structure. Density corresponding to VP1 is shown in blue, VP2 in green, VP3 in red, VP4 in yellow, and RNA segments in pink. Borders of a selected icosahedral asymmetric unit are in- dicated with a dashed triangle and positions of selected twofold, threefold, and fivefold symmetry axes are represented by an oval, triangle, and pen- tagon, respectively. (Scale bar, 5 nm.) (B) Cartoon representation of icosa- hedral asymmetric unit of rhinovirus 14 viewed from the inside of the capsid. The color coding of individual virus components is the same as in A. Positions of twofold, threefold, and fivefold symmetry axes are represented by an oval, triangle, and pentagon, respectively. (C) Two RNA octanucleotides that interact with each other and with VP2 subunits. Protein and RNA coloring is the same as in A. The cryo-EM density of the RNA segments is shown as a pink semitransparent surface. RNA bases and side chains of Trp38 of VP2 are shown in stick representation, in orange, and indicated with black arrow- heads. The position of a twofold symmetry axis is indicated with an oval. (D) Detail of stacking interaction between Gua2 from RNA segment and Trp38 of VP2. The cryo-EM densities of RNA and protein are shown as semitrans- parent surfaces in pink and gray, respectively. (E) Interaction between N ter- minus of VP1 and genome. Capsid proteins are shown in cartoon representation with the same coloring as in A. Cryo-EM densities of individual proteins are shown as semitransparent surfaces colored according to the chain they belong to. The density of the RNA genome is shown in gray. The blue arrow indicates the contact between the N terminus of VP1 and the genome. The position of Thr17, the first modeled residue from the N terminus of VP1, is indicated.

Pavel Plevka Research Group

Significance

Most rhinoviruses, which are the leading cause of the common cold, utilize intercellular adhesion molecule-1 (ICAM-1) as a receptor to infect cells. To release their genomes, rhinoviruses convert to acti- vated particles that contain pores in the capsid, lack minor capsid protein VP4, and have an altered genome organization. The binding of rhinoviruses to ICAM-1 promotes virus activation; however, the molecular details of the process remain unknown. Here, we present the structures of virion of rhinovirus 14 and its complex with ICAM-1 determined to resolutions of 2.6 and 2.4 Å, respectively. The cryo- electron microscopy reconstruction of rhinovirus 14 virions contains the resolved density of octanucleotide segments from the RNA ge- nome that interact with VP2 subunits. We show that the binding of ICAM-1 to rhinovirus 14 is required to prime the virus for activation and genome release at acidic pH. Formation of the rhinovirus 14– ICAM-1 complex induces conformational changes to the rhinovirus 14 capsid, including translocation of the C termini of VP4 subunits, which become poised for release through pores that open in the capsids of activated particles. VP4 subunits with altered conforma- tion block the RNA–VP2 interactions and expose patches of posi- tively charged residues. The conformational changes to the capsid induce the redistribution of the virus genome by altering the capsid–RNA interactions. The restructuring of the rhinovirus 14 cap- sid and genome prepares the virions for conversion to activated particles. The high-resolution structure of rhinovirus 14 in complex with ICAM-1 explains how the binding of uncoating receptors en- ables enterovirus genome release.

Hrebik, D. ; Fuzik, T. ; Gondova, M. ; Smerdova, L. ; Adamopoulos, A. ; Sedo, O.; Zdrahal, Z., and Plevka, P.: ICAM-1 induced rearrangements of capsid and genome prime rhinovirus 14 for activation and uncoating, PNAS 2021 Vol. 118 No. 19 e2024251118, https://doi.org/10.1073/pnas.2024251118