The incorporation of reovirus cell attachment protein σ1 into virions requires the N-terminal hydrophobic tail and the adjacent heptad repeat region

The N-terminal portion of the reovirus cell attachment protein σ1 has recently been shown to possess intrinsic oligomerization and virion-anchoring functions. Sequence analysis of the σl proteins of the three reovirus serotypes has revealed the presence of distinct structural domains within this region: a terminal hydrophobic tail, a hinge, and an extended coiled-coil. To probe the inter-relationship between the virion-anchoring function and the oligomerization function, we constructed two serotype 3 (T3) σ deletion mutants in SV40 expression vectors, one lacking the hydrophobic tail and the hinge, and the other lacking an adjacent region which constituted part of the coiled-coil. These mutants were (i) expressed in uninfected COS-1 cells and assayed for their ability to form oligomers, and (ii) expressed in type 1 (Ti) reovirus-infected COS-1 cells and assayed for their incorporability into progeny T1 virions. It was found that, whereas both truncated σ1 proteins were capable of forming stable oligomers, neither could be incorporated into virions. These observations, coupled with structural characteristics deduced from sequence analysis, are compatible with a model in which the hydrophobic tail is the bona fide σ1 anchorage domain and whose precise association with the virion spikes is dictated by an adjacent heptad repeat region linked to the former structure via a flexible hinge region.