Karel Škubník1, S. Kalynych1,
J. Nováček1, T. Füzik1, A. Přidal2, R. Paxton3,
J. de Miranda4,
P. Plevka1
1Structural Virology, Central European Inst. of
Technology, Masaryk Univ., Brno, Czech Republic
2Dept of Zoology, Fishery, Hydrobiology, and Apidology, Faculty of
Agronomy, Mendel Univ. in Brno, Brno, Czech Republic
3Inst. of Biology/Zoology, Martin Luther Univ. Halle-Wittenberg,
Halle, Germany
4Department of Ecology, Swedish University of Agricultural Sciences,
75651 Uppsala, Sweden
The worldwide population of western honeybee (Apis mellifera), the most economically important pollinator, is under pressure from habitat loss, environmental stress and diseases. Viruses from the family Iflaviridae cause lethal diseases in honeybees and other wild pollinators such as bumblebees (Bombus sp.). Two most important honeybee viruses from this family are deformed wing virus (DWV) and slow bee paralysis virus (SBPV).
Here we present the virion structures of DWV and SBPV determined by cryo-electron microscopy and X-ray crystallography. Capsid proteins VP3 of both of the viruses have C-terminal extension that fold into globular protruding (P)-domains exposed at the virion surfaces. Similar domains have not been previously observed in other viruses from the order Picornavirales. The P-domain contains conserved Asp-His-Ser catalytic triad. These residues may participate in receptor binding or provide the protease, lipase, or esterase activity required for the entry of the virus into the host cell. Furthermore, nucleotides from the DWV RNA genome interact with the capsid protein residues. Amino acids involved in the RNA binding are conserved among honeybee iflaviruses, suggesting a putative role of the genome in stabilizing the virion or facilitating capsid assembly.
Release of the virus genome from the capsid is a key step in infection of a cell. The acidic environment, that the virus encounter during cell entry, causes disruption of contacts between DWV capsid proteins, which results in expansion of the virus capsid and formation of pores around icosahedral threefold symmetry axes. This expansion may result in capsid disintegration or genome release though the channels. Understanding the genome release mechanism is a first step for development of compounds able to interupt this essential step in viral life-cycle.