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1.G.19.  The Nonenvelope Rotavirus Pore-forming Membrane Fusion Complex (Rotavirus MFC) Family

Fusion activity allowing rotavirus entry into a cell depends on the processing of several proteins, the primary ones being VP4 (which is cleaved into VP5 and VP8), and the secondary ones possibly being VP2 and VP6 (Gilbert and Greenberg 1997; Gilbert and Greenberg 1998). The spike protein VP4 is a principal component in the entry apparatus of rotavirus, a non-enveloped virus that causes gastroenteritis and kills ~440,000 children each year. Trypsin cleavage of VP4 primes the virus for entry by triggering a rearrangement that rigidifies the VP4 spikes. VP4 is cleaved to VP5 and VP8, and VP5 can permeabilize membranes (Denisova et al. 1999).  Dormitzer et al. 2004 determined the crystal structure, at 3.2 A resolution, of the main part of VP4 that projects from the virion, revealing a coiled-coil stabilized trimer. VP4 undergoes a rearrangement in which the oligomer reorganizes and each subunit folds back on itself, translocating a potential membrane-interaction peptide from one end of the spike to the other. This rearrangement resembles the conformational transitions of membrane fusion proteins of enveloped viruses.

Non-enveloped viruses of different types have evolved distinct mechanisms for penetrating a cellular membrane during infection. Rotavirus penetration appears to occur by a process resembling enveloped-virus fusion: membrane distortion linked to conformational changes in a viral protein. Evidence for such a mechanism comes from crystallographic analyses of fragments of VP4, the rotavirus-penetration protein, and infectivity analyses of structure-based VP4 mutants. Settembre et al. 2011 described the structure of an infectious rotavirus particle determined at 4.3 Å resolution. CryoEM image reconstruction permited a nearly complete trace of the VP4 polypeptide chain, including the positions of most side chains. It showed how the two subfragments of VP4 (VP8(*) and VP5(*)) retain their association after proteolytic cleavage.  It revealed multiple structural roles for the β-barrel domain of VP5(*), and specified interactions of VP4 with other capsid proteins.

 Enveloped viruses cross membranes through well-characterized membrane fusion mechanisms, but non-enveloped viruses, such as rotaviruses, require the destabilization of the host cell membrane by poorly understood processes. Elaid et al. 2014 identified, in the C-terminal region of the rotavirus glycoprotein VP7, a peptide that contains a putative membrane domain that folds into an amphipathic α-helix. Its structure was confirmed and the peptide inserted into membranes and permeabilized them although the native protein VP7 did not. In a hydrophobic environment, this helix has amphipathic properties characteristic of membrane-perforating peptides. Surprisingly, its structure varies from that of its counterpart in the structure of the native protein VP7, as was determined by X-ray crystalography. Such peptides could play a role by facilitating membrane crossing by non-enveloped viruses during cell infection (Gilbert and Greenberg 1998). The spike protein VP4 is a principal component in the entry apparatus of rotavirus, a non-enveloped virus that causes gastroenteritis and kills ~440,000 children each year. Trypsin cleavage of VP4 primes the virus for entry by triggering a rearrangement that rigidifies the VP4 spikes. VP4 is cleaved to VP5 and VP8, and VP5 can permeabilize membranes (Denisova et al. 1999).  Dormitzer et al. 2004 determined the crystal structure, at 3.2 A resolution, of the main part of VP4 that projects from the virion, revealing a coiled-coil stabilized trimer. VP4 undergoes a rearrangement in which the oligomer reorganizes and each subunit folds back on itself, translocating a potential membrane-interaction peptide from one end of the spike to the other. This rearrangement resembles the conformational transitions of membrane fusion proteins of enveloped viruses.

Non-enveloped viruses of different types have evolved distinct mechanisms for penetrating a cellular membrane during infection. Rotavirus penetration appears to occur by a process resembling enveloped-virus fusion: membrane distortion linked to conformational changes in a viral protein. Evidence for such a mechanism comes from crystallographic analyses of fragments of VP4, the rotavirus-penetration protein, and infectivity analyses of structure-based VP4 mutants. Settembre et al. 2011 described the structure of an infectious rotavirus particle determined at 4.3 Å resolution. CryoEM image reconstruction permited a nearly complete trace of the VP4 polypeptide chain, including the positions of most side chains. It showed how the two subfragments of VP4 (VP8(*) and VP5(*)) retain their association after proteolytic cleavage.  It revealed multiple structural roles for the β-barrel domain of VP5(*), and specified interactions of VP4 with other capsid proteins.

 Enveloped viruses cross membranes through well-characterized membrane fusion mechanisms, but non-enveloped viruses, such as rotaviruses, require the destabilization of the host cell membrane by poorly understood processes. Elaid et al. 2014 identified, in the C-terminal region of the rotavirus glycoprotein VP7, a peptide that contains a putative membrane domain that folds into an amphipathic α-helix. Its structure was confirmed and the peptide inserted into membranes and permeabilized them although the native protein VP7 did not. In a hydrophobic environment, this helix has amphipathic properties characteristic of membrane-perforating peptides. Surprisingly, its structure varies from that of its counterpart in the structure of the native protein VP7, as was determined by X-ray crystalography. Such peptides could play a role by facilitating membrane crossing by non-enveloped viruses during cell infection (Elaid et al. 2014).

References associated with 1.G.19 family:

Denisova, E., W. Dowling, R. LaMonica, R. Shaw, S. Scarlata, F. Ruggeri, and E.R. Mackow. (1999). Rotavirus capsid protein VP5* permeabilizes membranes. J. Virol. 73: 3147-3153. 10074166
Dormitzer, P.R., E.B. Nason, B.V. Prasad, and S.C. Harrison. (2004). Structural rearrangements in the membrane penetration protein of a non-enveloped virus. Nature 430: 1053-1058. 15329727
Elaid, S., S. Libersou, M. Ouldali, N. Morellet, B. Desbat, I.D. Alves, J. Lepault, and S. Bouaziz. (2014). A peptide derived from the rotavirus outer capsid protein VP7 permeabilizes artificial membranes. Biochim. Biophys. Acta. 1838: 2026-2035. 24746450
Gilbert, J.M. and H.B. Greenberg. (1997). Virus-like particle-induced fusion from without in tissue culture cells: role of outer-layer proteins VP4 and VP7. J. Virol. 71: 4555-4563. 9151849
Gilbert, J.M. and H.B. Greenberg. (1998). Cleavage of rhesus rotavirus VP4 after arginine 247 is essential for rotavirus-like particle-induced fusion from without. J. Virol. 72: 5323-5327. 9573313
Golantsova, N.E., E.E. Gorbunova, and E.R. Mackow. (2004). Discrete domains within the rotavirus VP5* direct peripheral membrane association and membrane permeability. J. Virol. 78: 2037-2044. 14747568
Settembre, E.C., J.Z. Chen, P.R. Dormitzer, N. Grigorieff, and S.C. Harrison. (2011). Atomic model of an infectious rotavirus particle. EMBO. J. 30: 408-416. 21157433