This family includes the major capsid protein of iridoviruses, chlorella virus and Spodoptera ascovirus, which are all dsDNA viruses with no RNA stage. This is the most abundant structural protein and can account for up to 45% of virion protein [1]. In Chlorella virus PBCV-1 the major capsid protein is a glycoprotein [2]. The four families of large eukaryotic DNA viruses, Poxviridae, Asfarviridae, Iridoviridae, and Phycodnaviridae, are referred to collectively as nucleocytoplasmic large DNA viruses or NCLDV. The virions of different NCLDV have dramatically different structures. The major capsid proteins of iridoviruses and phycodnaviruses, both of which have icosahedral capsids surrounding an inner lipid membrane, showed a high level of sequence conservation. A more limited, but statistically significant sequence similarity was observed between these proteins and the major capsid protein (p72) of ASFV, which also has an icosahedral capsid. It was surprising, however, to find that all of these proteins shared a conserved domain with the poxvirus protein D13L, which is an integral virion component thought to form a scaffold for the formation of viral crescents and immature virion [3].

1: Comparison of the major capsid protein genes, terminal redundancies, and DNA-DNA homologies of two New Zealand iridoviruses. Webby RJ, Kalmakoff J; Virus Res 1999;59:179-189. PMID:10082389

2: Characterization of the major capsid protein and cloning of its gene from algal virus PBCV-1. Graves MV, Meints RH; Virology 1992;188:198-207. PMID:1566573

3: Common origin of four diverse families of large eukaryotic DNA viruses. Iyer LM, Aravind L, Koonin EV; J Virol. 2001;75:11720-11734. PMID:11689653

4: The structure and evolution of the major capsid protein of a large, lipid-containing DNA virus. Nandhagopal N, Simpson AA, Gurnon JR, Yan X, Baker TS, Graves MV, Van Etten JL, Rossmann MG; Proc Natl Acad Sci U S A. 2002;99:14758-14763. PMID:12411581