Involved in pilus assembly, twitching motility and adhesion to host cells. Primes type IV pili (T4P) assembly and is required for inclusion of minor pilins PilV, PilW and PilX to the surface pili. Stabilizes assembled pilus fibers likely by antagonizing retraction mediated by PilT. Calcium-binding and calcium release by PilY1 seem to be essential for twitching motility and for regulation of pilus retraction dynamics of PilT (By similarity). Regulates surface-activated virulence possibly by acting as a surface-attachment mechanosensor (PubMed:25385640). Colocalizes with the T4P fraction when surface pili are present. Sheared surface fraction when PilV, PilW and PilX are also present. By the swarm surface growth conditions relative to liquid growth conditions (PubMed:22865844). By surface attachment (PubMed:25385640). Contains a region (residues 173-365) weakly homologous to eukaryotic mechanically sensitive Von Willebrand Factor a (VWFa). Loss of twitching motility, predesposes to autolysis and impairs the secretion of quinolones and pyocyanine, but on the other hand promotes growth in stationary phase and bacterial survival in murine airway infection models (PubMed:19054330). Has reduced congo red (CR) binding which is a measure of Pel exopolysaccharide (EPS) production, and shows both increased swarming motility and an altered swarming pattern compared to wild-type. Suppresses cyclic-di-GMP (c-di-GMP) degrading phosphodiesterase bifA deletion mutant's swarming defects and eliminates its hyperbiofilm formation although the global pools of c-di-GMP remain unchanged (PubMed:20233936). PilY1 single mutant does not exhibit any changes in rhamnolipid production or in levels of the flagellar subunit FliC (PubMed:22865844). Defective for surface-activated virulence toward amoebae (PubMed:25385640). Targeting PilY1-mediated mechanosensation as a global virulence regulator could be an attractive therapeutic strategy in the development of treatments for infections caused by P.aeruginosa. Overexpression of PilY1 represses swarming motility in a pilus-independent manner and the repression specifically requires the diguanylate cyclase (DGC) SadC but not other DGCs (PubMed:20233936). Increased c-di-GMP levels upon PilY1 overexpression in surface-grown cells, but only in the presence of SadC (PubMed:22865844). Belongs to the PilY1 family. Extended N-terminus.