SV40 AssemblyIn VivoandIn Vitro

Abstract

The Simian virus 40 (SV40) capsid is aT = 7dicosahedral lattice ∼45 nm in diameter surrounding the ∼5 kb circular minichromosome. The outer shell is composed of 360 monomers of the major capsid protein VP1, tightly bound in 72 pentamers. VP1 is a jellyroll β-barrel, with extending N- and C-terminal arms. The N-terminal arms bind DNA and face the interior of the capsid. The flexible C-arms tie together the 72 pentamers in three distinct kinds of interactions, thus facilitating the formation of aT = 7 icosahedron from identical pentameric building blocks. Assemblyin vivowas shown to occur by addition of capsomers around the DNA. We apply a combination of biochemical and genetic approaches to study SV40 assembly. Ourin vivoandin vitrostudies suggest the following model: one or two capsomers bind at a high affinity toses, the viral DNA encapsidation signal, forming the nucleation centre for assembly. Next, multiple capsomers attach concomitantly, at lower affinity, around the minichromosome. This increases their local concentration facilitating rapid, cooperative assembly reaction. Formation of the icosahedron proceeds either by gradual addition of single pentamers to the growing shell or by concerted assembly of pentamer clusters.