Background:The platform protein PilC and/or the alignment subcomplex PilMNOP may coordinate type IV pilus (T4P) extension/retraction ATPases. Results: Loss of PilC in a retraction-deficient background abolished assembly. C-terminal mutations were permissive for assembly but not twitching motility. Conclusion: PilC is essential for pilus biogenesis and twitching motility. Significance: Platform proteins are conserved essential components of T4P and type II secretion machines.