Polymerizes to form the thick (6.8 nm in diameter) rod of the pilus (also called fimbria). The rod is a right-handed helical cylinder with 3.28 PapA subunits per turn. Pili are polar filaments radiating from the surface of the bacterium to a length of 0.5-1.5 micrometers and numbering 100-300 per cell, and enable bacteria to colonize the epithelium of specific host organs. Strains of E.coli that cause infection of the human urinary tract produce pap-pili which are hair-like appendages consisting of about 1000 helically arranged subunits of the protein PapA. These pili mediate binding to digalactoside-containing glycolipids present on the epithelial cells which line the urinary tract. Belongs to the fimbrial protein family.