Self-assembly of purified polyomavirus capsid protein VP1

X-ray diffraction data from polyoma capsid crystals were phased by refinement of low-resolution starting models to obtain a self-consistent structural solution. The unexpected result that the hexavalent morphological unit is a pentamer shows that specificity of bonding is not conserved among the protein subunits in the icosahedrally symmetric capsid.

Protein subunits in the two layers of the disk of tobacco mosaic virus have very similar conformations. Much of the bonding between subunits is polar, including salt-bridge systems. Arginine residues play a prominent part here and elsewhere. Interactions within each layer involve groups whose contacts can be adjusted to allow the transition from disk to virus helix. Aromatic clusters within each molecule are linked in a hydrophobic girdle encircling each ring.