Cryo-EM Structure of a Novel Calicivirus, Tulane Virus

Tulane virus (TV) is a newly isolated cultivatable calicivirus that infects juvenile rhesus macaques. Here we report a 6.3 Å resolution cryo-electron microscopy structure of the TV virion. The TV virion is about 400 Å in diameter and consists of a T = 3 icosahedral protein capsid enclosing the RNA genome. 180 copies of the major capsid protein VP1 (~57 KDa) are organized into two types of dimers A/B and C/C and form a thin, smooth shell studded with 90 dimeric protrusions. The overall capsid organization and the capsid protein fold of TV closely resemble that of other caliciviruses, especially of human Norwalk virus, the prototype human norovirus. These close structural similarities support TV as an attractive surrogate for the non-cultivatable human noroviruses. The most distinctive feature of TV is that its C/C dimers are in a highly flexible conformation with significantly reduced interactions between the shell (S) domain and the protruding (P) domain of VP1. A comparative structural analysis indicated that the P domains of TV C/C dimers were much more flexible than those of other caliciviruses. These observations, combined with previous studies on other caliciviruses, led us to hypothesize that the enhanced flexibility of C/C dimer P domains are likely required for efficient calicivirus-host cell interactions and the consequent uncoating and genome release. Residues in the S-P1 hinge between the S and P domain may play a critical role in the flexibility of P domains of C/C dimers.