3-DIMENSIONAL STRUCTURE OF BACULOVIRUS-EXPRESSED NORWALK VIRUS CAPSIDS

The three-dimensional structure of the baculovirus-expressed Norwalk v irus capsid has been determined to a resolution of 2.2 nm using electr on cryomicroscopy and computer image processing techniques. The empty capsid, 38.0 nm in diameter, exhibits T=3 icosahedral symmetry and is composed of 90 dimers of the capsid protein. The striking features of the capsid structure are arch-like capsomeres, at the local and strict 2-fold axes, formed by dimers of the capsid protein and large hollows at the icosahedral 5- and 3-fold axes. Despite its distinctive archit ecture, the Norwalk virus capsid has several similarities with the str uctures of T=3 single-stranded RNA (ssRNA) viruses. The structure of t he protein subunit appears to be modular with three distinct domains: the distal globular domain (P2) that appears bilobed, a central stem d omain (P1), and a lower shell domain (S). The distal domains of the 2- fold related subunits interact with each other to form the top of the arch. The lower domains of the adjacent subunits associate tightly to form a continuous shell between the radii of 11.0 and 15.0 nm. No sign ificant mass density is observed below the radius of 11.0 nm. It is su spected that the hinge peptide in the adjoining region between the cen tral domain and the shell domain may facilitate the subunits adapting to various quasiequivalent environments. Architectural similarities be tween the Norwalk virus capsid and the other ssRNA viruses have sugges ted a possible domain organization along the primary sequence of the N orwalk virus capsid protein. It is suggested that the N-terminal 250 r esidues constitute the lower shell domain (S) with an eight-strand bet a-barrel structure and that the C-terminal residues beyond 250 constit ute the protruding (P1+P2) domains. A lack of an N-terminal basic regi on and the ability of the Norwalk virus capsid protein to form empty T =3 shells suggest that the assembly pathway and the RNA packing mechan isms may be different from those proposed for tomato bushy stunt virus and southern bean mosaic virus but similar to that in tymoviruses and comoviruses.