Regulated Proteolytic Processing within Mature Retroviral Capsids

Abstract

Capsid particles were prepared from equine infectious anemia virus (EIAV) as a model retrovirus for the human immunodeficiency virus (HIV). There is a stepwise cleavage of the nucleocapsid (NC) protein (pll) and integrase (IN) (p32) during incubation of EIAV capsids at 37°C in 10 mM Tris 1 mM EDTA (TE Buffer) at pH 7.6. The viral protease cleaves the NC protein after the first Cys residue of both conserved (C X2 C X4 H X4 C) regions. The p11 → p6 cleavage occurs at the first Cys array. The p6 is then cleaved at the second Cys array, resulting in three main peptide fragments appearing as a 4 KDa band on an SDS gel. The cleavage of a 6 KDa C-terminal fragment from IN starts when all the pll is cleaved to p6, and therefore occurs during the final fragmentation of the NC protein. Capsids from other retroviruses also show NC protein cleavage when incubated under similar conditions. It has been postulated that proteolytic processing of the NC protein occurs in vivo during the early stages of the viral life-cycle and may be required for replication.