Romero, E

Romero, E. step in the productive infection of a virus is its successful entry into a host cell. For many viruses, a single protein is implicated in this complex process that often involves multiple steps and multiple receptors. Such is the case with rotavirus, the major pathogen of infantile gastroenteritis accounting for nearly half a million deaths annually worldwide (8). Rotavirus, a member of the family, is a large icosahedral virus with a complex organization consisting of three concentric capsid layers that encapsidate 11 genomic double-stranded RNA segments (16, 41). The innermost capsid layer is composed of 120 copies of VP2 on a T=1 lattice (30), and 12 copies each of the RNA-dependent RNA polymerase VP1 (44) and the guanylyltransferase VP3 (6, 31) attach as heterodimeric complexes to the inner surface of this layer at the fivefold axial positions (41). The intermediate capsid layer is composed of 780 copies of VP6 arranged as 260 trimers on a T=13 lattice. The outermost capsid layer contains 780 copies of VP7, with the same icosahedral organization as the intermediate layer, and 120 copies of SCH 23390 HCl VP4, which interacts with VP6 and emanates as distinct bilobed spikes through the VP7 capsid layer. Antibody labeling and cryo-electron microscopy (cryo-EM) showed that the spikes on the surface of the triple-layered particle (TLP) are present as 60 dimers of VP4 located near the type II channels surrounding each fivefold vertex (40, 43). Although earlier studies implicated VP7 in the cell entry process (19, 42), subsequent studies have increasingly indicated that VP4 is the major player in this process. VP4 is implicated not only in cell attachment and penetration but also in hemagglutination, neutralization, virulence, and protease-enhanced infectivity of rotavirus (25, 32, 34). The latter phenomenon is particularly relevant considering that rotavirus replicates in the mature enterocytes of the small intestine, an environment rich in proteases. Proteolytic cleavage of VP4 primes the virus for efficient entry into cells (1, 17, 26). During proteolysis, VP4 (88 kDa) is cleaved into VP8* (28 kDa) and VP5* (60 SCH 23390 HCl kDa), and the cleavage products remain associated in the virion (18). Our recent structural and biochemical studies have shown that VP4 undergoes a conformational transition from a disordered SCH 23390 HCl to an ordered state upon trypsinization, and this transition appears to be responsible for trypsin-enhanced infectivity observed with rotavirus (10). The X-ray crystallographic structures of VP8* and VP5* have provided strong evidence that the distal globular domain of the VP4 spike is composed of VP8*, with the remaining body of the spike consisting of VP5* (12, 13, 43). A consensus opinion that has emerged from recent biochemical studies is that CD38 rotavirus entry into cells is a multistep process involving sialic acid (SA)-containing receptors in the initial cell attachment step and integrins such as v3, 41, and 21 during the subsequent postattachment steps (9, 22, 23, 45). In this process, the VP8* domain, which has a galectin fold, is involved in the interactions with SA, whereas VP5* is implicated in the interactions with integrins. Involvement of SA during rotavirus infection is not an essential step SCH 23390 HCl for all rotavirus strains. For the majority of rotavirus strains, including human rotaviruses, cell entry is SA independent (7). In these viruses, the majority of neutralizing monoclonal antibodies (MAbs) that recognize VP4 select mutations in VP5* (27, 28, 38), SCH 23390 HCl suggesting that cell entry is mediated mainly by VP5*. It also is thought that cell penetration of rotavirus may require a hydrophobic, fusion domain, which resides on the VP5* cleavage product. These hydrophobic regions could aid.