Catalyzes the N-deacetylation of poly-beta-1,6-N-acetyl-D-glucosamine (PNAG, also referred to as PIA), a biofilm adhesin polysaccharide. N-deacetylation is crucial for attachment of the polysaccharide to the bacterial cell surface; it leads to the introduction of positive charges in the otherwise neutral PIA polymer, allowing electrostatic interactions (By similarity). Attached to the cell surface. In strain COL, the gene icaC is interrupted by a natural frameshift. Belongs to the polysaccharide deacetylase family.