Sequence analysis of vitamin K dependent gamma-carboxylases (VKGC) revealed the presence of a novel domain, HTTM (Horizontally Transferred TransMembrane) in its N-terminus. In contrast to most known domains, HTTM contains four transmembrane regions. Its occurrence in eukaryotes, bacteria and archaea is more likely caused by horizontal gene transfer than by early invention. The conservation of VKGC catalytic sites indicates an enzymatic function also for the other family members.
Jiyao W et al.(2023). "The conserved domain database in 2023.",