Fig. 6: Model for the bidirectional motion of the CpaF motor.

A Superpositions comparing active sites of CpaF_AMPPNP open state with CpaF_ADP open’ and open states. CTDs only were used for each superposition. For clarity, bound Mg²⁺ has been omitted from the CpaF_ADP panels. B (Top panels) CpaF_AMPPNP shows how the open state (AMPPNP bound) has high affinity for ATP, which is then hydrolysed in the closed state (ADP/Mg²⁺ bound) with phosphate release. The open’ state has low affinity for ADP and is primed for its release. The sequence of this nucleotide hydrolysis cycle results in clockwise motion of the C2 symmetry axis mediated by large-scale conformational changes between NTD2/CTD_n+1 units. A clockwise motion is consistent with the extension of a right-handed pilus. (Bottom panels) Close similarity between the open’ state active site in CpaF_ADP and the open state in CpaF_AMPPNP is consistent with a model where the CpaF open’ state has suitable active site geometry to support ATP binding when ADP is bound in the open state. In this way, a counter-clockwise motion would be achieved. An allosteric modulator that may comprise the NTD0 is suggested to act as a gear switch controlling rotation direction possibly by binding and altering the geometry of active sites or nucleotide access and affinity. Other switches are viable such as phosphorylation.