Figure 1 Baculovirus phylogeny based on polyhedrin and granulin protein sequences. The deep division between the NPV and GV clades is clear. (Modified with permission from Zanotto et a/(1994) J. Invertebr. Pathol. 62:147-164.)
from approximately 100 to 180 kb. The nucleocapsids resemble those of the NPVs and are typically 30-60 nm in diameter and 200-300 nm long. The cylindrical body of the capsid appears to be constructed as a helical stacked ring structure that repeats every third ring. The ends of the capsid are different, both from each other and from the body of the capsid. Like the NPVs, it seems probable that GV virions occur in two morphologically distinct forms, a budded form that buds from infected cells (Fig. 2B), giving rise to extracellular virus, and an occluded form found within occlusion bodies formed inside infected cells. SDS-polyacrylamide gel electrophoresis analysis of GV virions suggest they contain in excess of 18 different polypeptides, with budded virions and occluded virions displaying different protein profiles.
GV occlusion bodies; often referred to as capsules or granules, have a roughly oval morphology, with a major axis diameter of 300-500 nm and a minor axis diameter of 120-300 nm. They typically contain a single nucleocapsid (Fig. 2C, D), although occasionally capsules with several nucleocapsids have been observed. Electron micrographs of GV capsules suggest they are surrounded by an outer layer which is likely to be similar to the calyx surrounding NPV occlusion bodies. The major protein of GV capsules is known as granulin. Granulin proteins have a size of 28-30 kDa and are related at the amino acid sequence level to the polyhedrin proteins of NPVs. High-resolution visualization of the matrix of the capsule reveals that it is highly crystalline (Fig. 2C, D). It is thought that multimers of granulin form the subunits of this crystalline lattice.
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