Molecular biology of astroviruses: selected highlights.
Human astrovirus, the prototype of the Astroviridae family, is a non-enveloped positive-strand RNA virus with distinctive morphology. Initially named for a characteristic 5-6 point star evident on the surface of faecally shed viral particles by direct electron microscopy, a recent study using cryoelectron microscopy and image reconstruction indicates that viral particles consist of a smoothly rippled, solid capsid decorated with short spikes. Mechanisms underlying the assembly of these viral particles have not been fully elucidated. However, studies of two full-length cDNA clones of human astrovirus serotype 1 suggest that capsid residue Thr227 plays a critical role in the assembly of infectious viral progeny. The development of a full-length clone (pAVIC) from which infectious RNA can be transcribed has also facilitated studies of the viral 3C-like serine protease, encoded in ORF1a. These studies demonstrate that the full-length ORF1a product (101 kDa) is processed in vitro to an N-terminal 64 kDa fragment and a C-terminal 38 kDa fragment. Mutation of the predicted catalytic triad inhibits proteolysis. In other studies based on modifications of pAVIC, preliminary evidence supports the feasibility of developing a reporter cell line to facilitate astrovirus detection.