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). Deletion of the icaADBCR genes leads to the inability to form biofilms, produce PIA or mediate N-acetylglucosaminyltransferase activity in vitro. Belongs to the IcaD family.