Structural Studies of Recombinant Norwalk Capsids

Norwalk virus is the major cause of epidemic viral gastroenteritis in humans. Attempts to grow this human virus in laboratory cell lines have been unsuccessful; however, the Norwalk virus capsid protein, when expressed in insect cells infected with a recombinant baculovirus, spontaneously assembles into virus-like particles. The x-ray crystallographic structure of these recombinant Norwalk particles has been determined to 3.4 A, using a 22-A electron cryomicroscopy structure as a phasing model. The recombinant capsids, 380 A in diameter, exhibit a T=3 icosahedral symmetry. The capsid is formed by 90 dimers of the capsid protein, each of which forms an arch-like capsomere. The capsid protein has two distinct domains-a shell (S) and a protruding (P) domain-that are connected by a flexible hinge. Although the S domain has a classical beta-sandwich fold, the structure of the P domain is unlike any other viral protein. One of the subdomains in the P domain formed by the most variable part of the sequence is located at the exterior of the capsid.