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. Major subunit of the fimbria. Self-associates. Forms filaments. Interacts with EcpD. Negatively regulated by H-NS. Positively regulated by IHF and EcpR (By similarity). Belongs to the EcpA/MatB fimbrillin family.