CAPSID ASSEMBLY IN A FAMILY OF ANIMAL VIRUSES PRIMES AN AUTOPROTEOLYTIC MATURATION THAT DEPENDS ON A SINGLE ASPARTIC-ACID RESIDUE

Maturation of noninfectious nodavirus provirions occurs by autoproteol ytic cleavage of most of the 180 copies of the alpha-protein that make up, the icosahedral capsid. This maturation, which is much slower tha n viral assembly, produces an infectious particle that is more stable than the provirion and makes viral uncoating thermodynamically distinc t from assembly, allowing assembly and (a time delayed) uncoating to o ccur under similar conditions. The results of structural, computationa l, and molecular genetic studies suggest that maturation depends both on intrasubunit strain, produced during assembly, and on a critical as partic acid residue. This residue lies in a hydrophobic pocket that is stabilized by intersubunit contacts. It is close to the scissile bond and exhibits an environmentally elevated pK(a). The apparent involvem ent of a single acidic residue in the hydrolytic cleavage of a peptide bond contrasts with the involvement of 2 such residues in acid protea ses.