A noncanonical cytochrome c stimulates calcium binding by PilY1 for type IVa pili formation

Abstract

Significance Type IVa pili (T4aP) are bacterial surface structures that function under different environmental conditions. In the machine for T4aP formation, a complex of minor pilins and PilY1 primes T4aP formation and is also present at the pilus tip mediating adhesion. Similar to several other bacterial adhesins, PilY1 depends on calcium binding for function. Here, we demonstrate that in Myxococcus xanthus, PilY1 at low levels of calcium depends on the accessory protein TfcP to bind calcium, thereby stabilizing the protein. TfcP is a noncanonical cytochrome c that does not participate in electron transport. Rather our data support that TfcP interacts transiently with PilY1 to stimulate calcium binding. In this way, TfcP expands the range of calcium levels under which T4aP functions.