Structural studies of recombinant Norwalk capsids

Norwalk virus is the major cause of epidemic viral gastroenteritis in human s. Attempts to grow this human virus in laboratory cell lines have been uns uccessful; however, the Norwalk virus capsid protein, when expressed in ins ect cells infected with a recombinant baculovirus, spontaneously assembles into virus-like particles. The x-ray crystallographic structure of these re combinant Norwalk particles has been determined to 3.4 Angstrom, using a 22 -Angstrom electron cryomicroscopy structure as a phasing model. The recombi nant capsids, 380 Angstrom in diameter, exhibit a T = 3 icosahedral symmetr y. The capsid is formed by 90 dimers of the capsid protein, each of which f orms 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 hing e. 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 a t the exterior of the capsid.