Herpesvirus Assembly and Egress

Abstract

Herpesvirus particles consist of four morphologically distinct structures, the core, capsid, tegument, and envelope. The inner nucleoprotein core comprising the linear double-stranded DNA genome is included in an icosahedral (T=16) capsid shell of 150 hexons and 12 pentons. The capsid is surrounded by a layer of proteinaceous material designated the tegument which, in turn, is enclosed in an envelope of host cell-derived lipids containing virus-encoded (glyco)proteins. Whereas capsid formation in the nuclei of infected cells is understood in some detail, the mechanisms of tegumentation and envelopment and the intracellular compartments involved have long been disputed. This review focuses on recent findings that demonstrate a rather complex process of herpesvirus maturation including primary envelopment of capsids by budding at the inner leaflet of the nuclear membrane and translocation of capsids into the cytoplasm after loss of the primary envelope by fusion with the outer leaflet of the nuclear membrane. Subsequently, final tegumentation occurs in the cytoplasm and tegumented capsids obtain their final envelope by budding into vesicles of the trans-Golgi network. Tegumentation and envelopment are driven by specific protein-protein interactions that appear, at least in cultured cells, to exhibit a remarkable redundancy.