This domain is found in the archaeal protein O6-alkylguanine-DNA alkyltransferase (also known as Methylated DNA-protein cysteine methyltransferase MGMT), adopt a secondary structure consisting of a three stranded antiparallel beta-sheet and three alpha helices. This entry represents the N-terminal ribonuclease H-like domain, associated with 6-O-methylguanine DNA methyltransferase activity. The exact function has not, as yet, been defined, though it has been postulated that they confer thermostability to the archaeal protein [1].

1: Hyperthermostable protein structure maintained by intra and inter-helix ion-pairs in archaeal O6-methylguanine-DNA methyltransferase. Hashimoto H, Inoue T, Nishioka M, Fujiwara S, Takagi M, Imanaka T, Kai Y; J Mol Biol. 1999;292:707-716. PMID:10497033