ABSTRACT
The Tetraviridae are a family of small, non-enveloped, insect RNA viruses consisting of one or two single-stranded, positive-sense genomic RNAs encapsidated in an icosahedral capsid with T=4 symmetry. Tetravirus procapsids undergo maturation when exposed to a low pH environment in vitro. While the structural biology of the conformational changes that mediate acid-dependent maturation is well understood, little is known about the significance of acid-dependent maturation in vivo. To address this question, the capsid-coding sequence of the tetravirus Helicoverpa armigera stunt virus was expressed in Saccharomyces cerevisiae cells. Virus-like particles were shown to assemble as procapsids that matured spontaneously in vivo as the cells began to age. Growth in the presence of hydrogen peroxide or acetic acid, which induced apoptosis or programmed cell death in the yeast cells, resulted in virus-like particle maturation. The results demonstrate that assembly-dependent maturation of tetravirus procapsids in vivo is linked to the onset of apoptosis in yeast cells. We propose that the reduction in pH required for tetraviral maturation may be the result of cytosolic acidification, which is associated with the early onset of programmed cell death in infected cells.
