Matt McCallum, a PhD student in the Howell lab who is working on our collaborative CIHR funded project on structure of the type IV pilus system, determined the structure of the T4P polymerase, PilB, from Geobacter at ~2.3 A. Combined with our earlier insights about interactions of PilB with the platform protein PilC (Takhar et al., 2013), and the recent cryoelectron tomography structure of the T4P system from Myxococcus solved by the Jensen lab (Chang et al., 2016), we propose a mechanism for motor function that explains how pilins are added to the growing pilus by PilB, and by extension, how they are removed by PilT, the retraction ATPase. The paper has been posted on the BioRXiv preprint server: http://biorxiv.org/content/biorxiv/early/2016/10/24/082750.full.pdf