The heptad repeats region is essential for AcMNPV P10 filament formation and not the proline-rich or the C-terminus basic regions

In cultured cells the baculovirus P10 protein forms two independent intracellular structures that play separate roles in occlusion body maturation and their release by nuclear disintegration

P10 is a small, abundant baculovirus protein that accumulates to high levels in the very late stages of the infection cycle. It is associated with a number of intracellular structures and implicated in diverse processes from occlusion body maturation to nuclear stability and lysis. However, studies have also shown that it is non-essential for virus replication, at least in cell culture. Here, we describe the use of serial block-face scanning electron microscopy to achieve high-resolution 3D characterisation of P10 structures within Trichoplusia ni TN-368 cells infected with Autographa californica multiple nucleopolyhedrovirus. This has enabled unparalleled visualisation of P10 and determined the independent formation of dynamic perinuclear and nuclear vermiform fibrous structures. Our 3D data confirm the sequence of ultrastructural changes that create a perinuclear cage from thin angular fibrils within the cytoplasm. Over the course of infection in cultured cells, the cage remodels to form a large polarised P10 mass and we suggest that these changes are critical for nuclear lysis to release occlusion bodies. In contrast, nuclear P10 forms a discrete vermiform structure that was observed in close spatial association with both electron dense spacers and occlusion bodies; supporting a previously suggested role for P10 and electron dense spacers in the maturation of occlusion bodies. We also demonstrate that P10 hyper-expression is critical for function. Decreasing levels of p10 expression, achieved by manipulation of promoter length, correlated with reduced P10 production, a lack of formation of P10 structures and a concomitant decrease in nuclear lysis.

High-resolution 3D electron microscopy has revealed the complexity of structures formed by P10, a small 10kDa protein that accumulates to very high levels in baculovirus-infected cells. We demonstrate the formation and presence of two distinct, possibly unique, P10 structures that account for the diverse roles associated with this small protein. In the cytoplasm, a peri-nuclear cage-like structure matured into a polarised mass of P10. Remodelling of the cage provides evidence for a mechanism to effect nuclear lysis and release of occlusion bodies to promote dispersal. Over a similar time period, an independent vermiform P10 structure forms and matures within the cell nucleus. It is widely known that in the absence of P10, occlusion bodies do not fully mature. Our data suggest a mechanism for occlusion body maturation with P10 facilitating the envelopment of occlusion bodies with electron dense spacers. The P10 structures formed require vast quantities of P10 protein providing a rationale for the hyper-expression of this hitherto obscure viral protein.

Phylogenetic analysis of Bombyx mori nucleopolyhedrovirus polyhedrin and p10 genes in wild isolates from Guangxi Zhuang Autonomous Region, China

Bombyx mori nucleopolyhedrovirus (BmNPV) is a severe pathogen that seriously impacts the sericulture industry. In this study, 45 wild BmNPV isolates were collected from different silkworm-raising regions in China's Guangxi Zhuang Autonomous Region. Two highly expressed very late genes from each isolate, polyhedrin and p10, were sequenced and subjected to phylogenetic analysis. The polyhedrin gene was found to be highly conserved, while the p10 gene was more variable frequently harboring point mutations and displaying variations in codon use without obvious codon bias. The BmNPV isolates from Guangxi were separated into three main clades, I, II, and III, according to the p10 gene phylogenetic tree. The geographical distribution of clade I isolates in Guangxi showed a concentrated pattern and that of clade II isolates showed a connected pattern. Clade III isolates were irregularly scattered throughout Guangxi. Local transmission of this pathogen clearly occurred in the silkworm-raising regions in Guangxi. This study may provide some data on BmNPV transmission in the silkworm-raising regions and be helpful in devising strategies for the prevention and control of BmNPV disease.