The PCI (for Proteasome, COP9, Initiation factor 3) domain (sometimes alsoreferred to as the PINT domain, for Proteasome subunits, Int-6, Nip-1, andTrip-15) is present in six different subunits of 26 proteasome lid, COP9signalosome (CSN) and eukaryotic translation initiation factor-3 (eIF3)complexes, as well as in subunits of certain other multiprotein complexes. ThePCI domain mediates and stabilizes protein-protein interactions within thecomplexes. The role of the PCI domains is most likely that of a scaffold forthe other complex subunits and other binding partners. The PCI domain couldplay a role as a universal binding domain supporting intra-complexinteractions as well as recruitments of additional ligands.PCI is an ~190-amino acid domain, not well conserved in its primary sequence,usually located near the C terminus of the protein. It does not contain anyinvariant residues or any conserved pattern of charged residues that wouldsuggest a catalytic activity. The PCI domain is comprised of two subdomainsthat are intimately connected, an N-terminal helical bundle (HB) subdomain anda C-terminal globular winged helix (WH) subdomain. The C-terminal half of the PCI domain is much better conserved than the N-terminalhalf.