Fig. 7: Models for CpaF engagement and energisation of CpaG and CpaH.

A Schematic of the canonical TadPS positioning CpaF in the equivalent position of PilB in the type 4 pilus system⁴⁷. Both canonical TadPS and C. crescentus specific Cpa nomenclature is shown. For clarity some canonical TadPS components are omitted including TadV/CpaA and TadZ/CpaE. Single subunits of CpaG and CpaH are depicted as silhouettes based on Alphafold modelling. E and F stand for TadE/CpaJ and TadF/CpaK. Recently it has been suggested that TadG is pilin related and will be positioned at the pilus tip⁶². B (Top) Rotary model where a CpaG and CpaH heterodimer, depicted as footprints, has one high-affinity binding interface per CpaF asymmetric unit. As the CpaF C2 symmetry axis rotates due to nucleotide cycling, the CpaG/CpaH heterodimer sequentially binds and unbinds to this interface resulting in a circular trajectory that is coupled to the spooling of pilins. Given the C2 symmetry, up to two CpaG/CpaH heterodimers could bind and rotate around a single hexamer. B (Bottom) Gyratory model based on three CpaG/CpaH heterodimers bound to one CpaF hexamer. This stoichiometry is supported in the T2SS PulF/GspF⁴⁸. As the CpaF C2 symmetry axis rotates, the surface topography at the CpaG/CpaH interface undergoes a complex sequence of conformational changes including the Central Loop moving between high (CL_H), medium (CL_M) and low (CL_L) elevations. The transmission of these conformational changes to CpaG/CpaH would support a gyratory-like movement in the membrane-spanning domains that would be coupled to the spooling of pilins.