Part of the ecpRABCDE operon, which encodes the E.coli common pilus (ECP). ECP is found in both commensal and pathogenic strains and plays a dual role in early-stage biofilm development and host cell recognition. Tip pilus adhesin, which is required for assembly of EcpA into fibers. Forms polymers. Interacts with EcpA. Polymerized EcpD is normally incorporated within the tip of ECP, but it can also form functional fimbria independent of EcpA. Negatively regulated by H-NS. Positively regulated by IHF and EcpR. Belongs to the EcpD/MatE family.