Binding of reovirus to receptor leads to conformational changes in viral capsid proteins that are reversible upon virus detachment

A conformational change was detected in reovirus upon its attachment to mouse L fibroblasts. Specifically, the capsid proteins of cell-bound virions became more resistant to pepsin digestion. Similar observations were made using glutaraldehyde-fixed cells or plasma membranes instead of live cells, indicating that virus internalization was not necessary for this effect. This conformational change was totally reversible, since bound virions reverted back to the pepsin-sensitive state upon release from the cell surface. Not unexpectedly, a conformational change was also detected in the reovirus cell attachment protein σ1 when it alone bound to cells. The alteration was mapped, by deletion mutagenesis, to a region proximal to the N-terminal (virion-anchoring) end of the protein and was also found to be reversible. Structural changes in σ1 were also detectable following its interaction with sialic acid (conjugated to bovine serum albumin) shown previously to be the minimal receptor determinant recognized by reovirus. These results suggest that upon cell attachment, a signal is transmitted from the C-terminal receptor-binding region of σ1 to the N terminus in a ripple-like progression that eventually leads to conformational changes in the other reovirus capsid proteins. An altered conformational state may be necessary for subsequent viral entry and programmed disassembly of viral capsids inside susceptible cells.