1.J.1 The Archaeal Virus-Associated Pyramid (A-VAP) Family
Some viruses of Archaea use an unusual egress mechanism that involves the formation of virus-associated pyramids (VAPs) on the host cell surface. At the end of the infection cycle, these structures open outward and create apertures through which mature virions escape from the cell. Quax et al. (2011) described the structure and composition of VAPs formed by the Sulfolobus islandicus rod-shaped virus 2 (SIRV2) in cells of its hyperthermophilic archaeal host. They found that the VAPs are stable and autonomous assemblies that can be isolated from membranes of infected cells and purified without affecting their structure. The purified VAPs are heterogeneous in size, reflecting the dynamics of VAP development in a population of infected cells; however, they have a uniform geometry, consisting of seven isosceles triangular faces forming a baseless pyramid. Biochemical and immunoelectron microscopy analyses revealed that the 10-kDa P98 protein encoded by the SIRV2 virus is the sole component of the VAPs. The VAPs were produced in Sulfolobus acidocaldarius and Escherichia coli by heterologous expression of the SIRV2-P98 gene. The results confirm that P98 is the only constituent of the VAPs and demonstrate that no other viral protein is involved in the assembly of pyramids. P98 was able to produce stable structures under conditions ranging from moderate to extremely high temperatures (80 °C) and from neutral to extremely acidic pH (pH 2), demonstrating another remarkable property of this exceptional viral protein.
A-VAPs belong to the PHA01757 superfamily of CDD. They are formed in archaeal viruses, SIRV1, SIRV2, Sulfolobus turreted icosahedral virus (STIV) and Stygiolobus rod-shaped virus (SRSV). The protein is also homologous to an internal segment/residues 47-144) of the phage tail length tape measure protein of Candidatus Nitrospira defluvii (length, 871aas; D8P844).
The reaction catalyzed by the SIRV2 P98 pyramids is:
virus particle (cytoplasm) _P98 pyrimidal structure → virus particle (external medium)