Energetic communication between functional sites of the gene-3-protein during infection by phage fd.

To initiate infection of Escherichia coli, phage fd uses its gene-3-protein (G3P) to bind first to an F pilus and then to the TolA protein at the cell surface. G3P is normally auto-inhibited because a tight interaction between the two N-terminal domains N1 and N2 buries the TolA binding site. Binding of N2 to the pilus activates G3P by initiating long-range conformational changes that are relayed to the domain interface and to a proline timer. We discovered that the 23-28 loop of the N1 domain is critical for propagating these conformational signals. The analysis of the stability and the folding dynamics of G3P variants with a shortened loop combined with TolA interaction studies and phage infection experiments reveal how the contact between the N2 domain and the 23-28 loop of N1 is energetically linked with the interdomain region and the proline timer and how it affects phage infectivity. Our results illustrate how conformational transitions and prolyl cis/trans isomerization can be coupled energetically and how conformational signals to and from prolines can be propagated over long distances in proteins.