These proteins are essential for the biogenesis of mature CS3 pili, but not for synthesis of the CS3 pilin subunit. A 97 kDa form of the protein is thought to be due to post-translational processing of isoform 104 kDa. Suppression of amber codon 754 seems to be amino acid specific; a supE suppressor (usually Gln) permits function, whereas a supF suppressor (Tyr) does not. Requires the suppression or readthrough of an internal amber codon in position 754. Belongs to the fimbrial export usher family.