Fimbriae (also called pili), polar filaments radiating from the surface of the bacterium to a length of 0.5-1.5 micrometers and numbering 100-300 per cell, enable bacteria to colonize the epithelium of specific host organs. PapH seems to anchor the pilus to the bacterial cell. In addition the stoichiometric relationship between PapH and PapA determines the pilus length. 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.