PARTICLE POLYMORPHISM CAUSED BY DELETION OF A PEPTIDE MOLECULAR SWITCH IN A QUASIEQUIVALENT ICOSAHEDRAL VIRUS

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.