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 (By similarity). 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. Adhesin for human tissue specifically recognizing a host receptor localized or enriched on basolateral epithelial cell surfaces. Binds host integrins in an calcium-dependent manner in vitro and this interaction may be employed by the bacterium to mediate host epithelial cell binding in vivo. Interacts (via C-terminal 532-1163) 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 (PubMed:20331639, PubMed:8899718). Sheared surface fraction when PilV, PilW and PilX are also present (By similarity). Loss of surface T4P which is not due to the lack of major pilus structural subunit PilA (PubMed:20080557, PubMed:8899718, PubMed:20331639). Lacks measurable twitching motility (PubMed:20080557, PubMed:8899718). T4P fibers are restored when pilus retraction ATPase pilT is deleted at the same time. Does not bind to the injured regions of human ciliated airway epithelium (HAE) during bacterial infection on the contrary to the wild-type which preferentially binds to the injured regions of the HAE (PubMed:20331639). Residues 600-608 comprise a calcium-binding site which together with the other one (residues 851-859) seems important for interaction with integrins of the host. Belongs to the PilY1 family.