The twist of a peptide bond in a viral protein starts a timer for infection, according to Barbara Eckert, Franz Schmid, and colleagues (Universität Bayreuth, Germany). To be successful, the virus must find its host's receptor before this timer goes off.
Bacterial infection by the phage fd is initiated by its gene-3-protein (G3P), which is folded into a protected conformation that is stable, robust, and fairly resistant to degradation, but not infective. Before infection, G3P must be opened via an initial contact with the bacterial F pilus to expose the binding site for its ultimate receptor, TolA. This opening converts a glutamine–proline bond in the hinge between the two binding domains from the cis to the trans conformation. This (nonnative) trans proline locks G3P in the open, infectious form.
The trans proline bond takes several minutes to isomerize spontaneously back to cis. “The back reaction...
