This is a domain found in STT3 present in P. furiosus. In P. furiousus, STT3 is an Oligosaccharyltransferase which catalyzes the transfer of a heptasaccharide, containing one hexouronate and two pentose residues, onto peptides in an Asn-X-Thr/Ser-motif-dependent manner. This domain is inserted into the central core (CC) domain and hence is referred to as the Insert (IS) domain. This IS domain contains a disulphide bond [1].

1: Structure-guided identification of a new catalytic motif of oligosaccharyltransferase. Igura M, Maita N, Kamishikiryo J, Yamada M, Obita T, Maenaka K, Kohda D; EMBO J. 2008;27:234-243. PMID:18046457