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 (PubMed:25389296). 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). Interacts (via C-terminus) with host integrins alpha-V/beta-3 (ITGAV/ITGB3) and alpha-V/beta-5 (ITGAV/ITGB5). Colocalizes with the T4P fraction when surface pili are present (By similarity). Sheared surface fraction when PilV, PilW and PilX are also present (PubMed:25389296). Pilus assembly severely impaired. Retraction-deficient. Prevents incorporation of PilE into pili. Residues 598-606 comprise a calcium-binding site which together with the other one (residues 849-857) seems important for interaction with integrins of the host. Belongs to the PilY1 family. Extended N-terminus.