Cyclases involved in the biosynthesis of lantibiotics, and similar proteins. LanC is the cyclase enzyme of the lanthionine synthetase. Lanthionine is a lantibiotic, a unique class of peptide antibiotics. They are ribosomally synthesized as a precursor peptide and then post-translationally modified to contain thioether cross-links called lanthionines (Lans) or methyllanthionines (MeLans), in addition to 2,3-didehydroalanine (Dha) and (Z)-2,3-didehydrobutyrine (Dhb). These unusual amino acids are introduced by the dehydration of serine and threonine residues, followed by thioether formation via addition of cysteine thiols, catalysed by LanB and LanC or LanM. LanC, the cyclase component, is a zinc metalloprotein, whose bound metal has been proposed to activate the thiol substrate for nucleophilic addition. A related domain is also present in LanM and other pro- and eukaryotic proteins with poorly characterized functions.