Plays an essential role in forming the main structure of the narrow T4P pili that participates in twitching motility. The single N-terminal transmembrane is initially involved in the correct localization to the inner membrane. Once the leader sequence cleaved, this region plays a role in multimerization and protein-protein interactions in the periplasm and the outer membrane. Glycosylated. Deletion mutants show a defect in twitching motility while DNA uptake seems not affected.