The Role of Tape Measure Protein in Nucleocytoplasmic Large DNA Virus Capsid Assembly

Abstract

Nucleocytoplasmic large DNA viruses (NCLDVs) are a group of large viruses that infect a wide range of hosts, from animals to protists. These viruses are grouped together in NCLDV based on genomic sequence analyses. They share a set of essential genes for virion morphogenesis and replication. Most NCLDVs generally have large physical sizes while their morphologies vary in different families, such as icosahedral, brick, or oval shape, raising the question of the possible regulatory factor on their morphogenesis. The capsids of icosahedral NCLDVs are assembled from small building blocks, named capsomers, which are the trimeric form of the major capsid proteins. Note that the capsids of immature poxvirus are spherical even though they are assembled from capsomers that share high structural conservation with those icosahedral NCLDVs. The recently published high resolution structure of NCLDVs, Paramecium bursaria Chlorella virus 1 and African swine fever virus, described the intensive network of minor capsid proteins that are located underneath the capsomers. Among these minor proteins is the elongated tape measure protein (TmP) that spans from one icosahedral fivefold vertex to another. In this study, we focused on the critical roles that TmP plays in the assembly of icosahedral NCLDV capsids, answering a question raised in a previously proposed spiral mechanism. Interestingly, basic local alignment search on the TmPs showed no significant hits in poxviruses, which might be the factor that differentiates poxviruses and icosahedral NCLDVs in their morphogenesis.

The 4.4 Å structure of the giant Melbournevirus virion belonging to the Marseilleviridae family

“…S11). However, Xiao et al reported in PBCV-1 and CroV that one of six MCP trimers of the asymmetric unit, which located at the corner of the pentasymmetron, was exceptionally rotated 60° (Xiao et al 2017, Xian et al 2020. They called this MCP trimer array of the asymmetric unit the "golf club motif".…”

“…The pentasymmetron assembly of Melbournevirus shares the MCP orientation ("golf club") motif displayed in CroV (Xiao et al 2017) and PBCV-1 (Xian et al 2020), in addition to the trisymmetron (Fig. 5).…”

“…In the motif, the corner, Pnd (n=1,2,3,4,5), of the MCP trimer in the asymmetric unit shows a 60° rotation to match that of the adjacent asymmetric unit. This is curious as Xian et al hypothesise this orientational variance in the pentasymmetron asymmetric unit to be caused by the tape measure protein (Xian et al 2020), which Melbournevirus does not exhibit, but instead appears to use the scaffold array (Fig. 2) and some other mCPs which were originally reported by Chihara et al (2021).…”

“…However, manual data collection with the HVEM microscope restricted the number of images collected, and the current resolution is limited to ~8 Å after using a capsid-focussed mask. The tokyovirus cryo-EM map elucidated the novel capsid protein network of the giant virus including the scaffold proteins which could assist in the formation of the characteristic internal membrane extrusion at the fivefold vertices of the capsid and function in a similar manner to a tape measure protein (Xian et al, 2020). Furthermore, it suggested a glycosylated cap protein was sitting on the MCP trimer (Chihara et al, 2021).…”

A novel capsid protein network allows the characteristic inner membrane structure of Marseilleviridae giant viruses

“…The tape-measure protein is a long filamentous protein that extends from a pentasymmetron to an adjacent pentasymmetron along the trisymmetron edge. It was proposed that the length of this tape-measure protein determines the capsid size 23 .…”