Necessary for the synthesis of poly-beta-1,6-N-acetyl-D-glucosamine (PNAG, also referred to as PIA), a biofilm adhesin polysaccharide. Is required for full IcaA N-acetylglucosaminyltransferase activity (By similarity). In strain COL, the gene icaC is interrupted by a natural frameshift. Belongs to the IcaD family.