A repurposed, non-canonical cytochrome c, chaperones calcium binding by PilY1 for type IVa pili formation

Type IVa pili (T4aP) are versatile bacterial cell surface structures that undergo extension/adhesion/retraction cycles powered by the cell envelope-spanning T4aP machine. In this machine, a complex composed of four minor pilins and PilY1 primes T4aP extension and is also present at the pilus tip mediating adhesion. Similar to many other bacteria, Myxococcus xanthus contains multiple minor pilins/PilY1 sets that are incompletely understood. Here, we report that minor pilins and PilY1 (PilY1.1) of cluster_1 form priming and tip complexes contingent on a non-canonical cytochrome c (TfcP) with an unusual His/Cys heme ligation and calcium. We provide evidence that TfcP is unlikely to participate in electron transport and has been repurposed to promote calcium binding by PilY1.1 at low calcium concentrations, thereby stabilising PilY1.1 and enabling T4aP function in a broader range of calcium concentrations. These results identify a novel function of cytochromes c and illustrate how incorporating an accessory factor expands the environmental range under which the T4aP system functions.