The Astrovirus Capsid: A Review

Abstract

Astroviruses are enterically transmitted viruses that cause infections in mammalian and avian species. Astroviruses are nonenveloped, icosahedral viruses comprised of a capsid protein shell and a positive-sense, single-stranded RNA genome. The capsid protein undergoes dramatic proteolytic processing both inside and outside of the host cell, resulting in a coordinated maturation process that affects cellular localization, virus structure, and infectivity. After maturation, the capsid protein controls the initial phases of virus infection, including virus attachment, endocytosis, and genome release into the host cell. The astrovirus capsid is the target of host antibodies including virus-neutralizing antibodies. The capsid protein also mediates the binding of host complement proteins and inhibits complement activation. Here, we will review our knowledge on the astrovirus capsid protein (CP), with particular attention to the recent structural, biochemical, and virological studies that have advanced our understanding of the astrovirus life cycle.

Keywords: antigen; astrovirus; capsid; structure; virus entry; virus exit.

Figure 1

Replication cycle of human astroviruses (adapted from [2]). (a) Synthesis of the 90 kDa human astroviruses (HAstV) capsid protein (VP90) from viral subgenomic RNA (sgRNA); (b) Assembly of VP90 proteins (180 copies) with viral genomic RNA into HAstV particles; (c) Caspase-mediated cleavage of VP90 C-termini to form VP70 and subsequent release of immature HAstV particles; (d) Extracellular protease cleavage of immature HAstV particles to form mature, infectious HAstV particles. In cell culture, trypsin is used to produce mature, infectious HAstV particles; (e) Extracellular HAstV particles induce host antibody production and inhibit host complement activation; (f) Attachment and clathrin-dependent endocytosis of HAstV particles; (g) Uncoating of the virus genome in the late endosome. Extracellular signal-regulated kinase (ERK1/2) and phosphoinositide 3-kinase (PI3K) are activated during HAstV binding or entry into the cell.

Figure 2

Schematic of HAstV-1 capsid protein (CP) domain structure and proteolytic processing (adapted from [14]). The CP is colored by structural domains, with the inner core domain in blue, the outer core domain in yellow, and the spike domain in red. Putative caspase and trypsin cleavage sites are specified with white and orange arrows, respectively. Putative trypsin cleavage sites (arginine and lysine residues) are noted only in the outer core region. The yellow hatched region in VP34 represents a region that is cleaved at multiple sites by trypsin but likely remains structured as part of the core domain in the mature HAstV virion. Putative caspase cleavage sites were previously predicted [12].

Figure 3

CryoEM and crystal structures of the astrovirus CP. (a) Immature T = 3 HAstV model made by fitting CP core and spike crystal structures into the immature HAstV virion cryo-EM map [22]; (b) Mature T = 3 HAstV model made by fitting CP core and spike crystal structures into the mature (trypsin-digested) HAstV virion cryo-EM map [22]; (c) Hepatitis E Virus (HEV) CP structure fit into the HEV T = 3 virus-like particle cryo-EM maps [25]; (d) Crystal structure of the HAstV-1 CP core domain [14]; (e) Crystal structure of the dimeric HAstV-1 CP spike domain [14]; (f) Crystal structure of the dimeric turkey astrovirus 2 (TAstV-2) CP spike domain [26]. All CP domains are colored as in Figure 2.

Figure 4

Antibody PL-2 binding site, complement C1q binding site, and putative receptor binding site on the HAstV surface. (a) Crystal structure of the HAstV-2 CP spike/scFv PL-2 complex [28]; (b) Footprint of the antibody PL-2 epitope (in cyan) on the HAstV-1 surface [28]; (c) Location of the complement C1q binding peptide CP1 (CP residues 79–108) (in cyan) on the HAstV-1 surface [45]; (d) Location of a putative receptor binding site P site or Site 1 (in cyan) on the HAstV-1 surface [28,29].