Xf. Dong et al., PARTICLE POLYMORPHISM CAUSED BY DELETION OF A PEPTIDE MOLECULAR SWITCH IN A QUASIEQUIVALENT ICOSAHEDRAL VIRUS, Journal of virology, 72(7), 1998, pp. 6024-6033
The capsid of flock house virus is composed of 180 copies of a single
type of coat protein which forms a T=3 icosahedral shell. High-resolut
ion structural analysis has shown that the protein subunits, although
chemically identical, form different contacts across the twofold axes
of the virus particle, Subunits that are related by icosahedral twofol
d symmetry form flat contacts, whereas subunits that are related by qu
asi-twofold symmetry form bent contacts, The flat contacts are due to
the presence of ordered genomic RNA and an ordered peptide arm which i
s inserted in the groove between the subunits and prevents them from f
orming the dihedral angle observed at the bent quasi-twofold contacts.
We hypothesized that by deleting the residues that constitute the ord
ered peptide arm, formation of flat contacts should be impossible and
therefore result in assembly of particles with only bent contacts. Suc
h particles would have T=1 symmetry, To test this hypothesis we genera
ted two deletion mutants in which either 50 or 31 residues were elimin
ated from the N terminus of the coat protein, We found that in the abs
ence of residues 1 to 50, assembly was completely inhibited, presumabl
y because the mutation removed a cluster of positively charged amino a
cids required for neutralization of encapsidated RNA. When the deletio
n was restricted to residues 1 to 31, assembly occurred, but the produ
cts were highly heterogeneous. Small bacilliform-like structures and i
rregular structures as well as wild-type-like T=3 particles were detec
ted. The anticipated T=1 particles, on the other hand, were not observ
ed. We conclude that residues 20 to 30 are not critical for formation
of flat protein contacts and formation of T=3 particles, However, the
N terminus of the coat protein appears to play an essential role in re
gulating assembly such that only one product, T=3 particles, is synthe
sized.