Transglutaminase 1 is conserved in amphibians and required for membrane-associated transglutaminase activity in the epidermis of clawed frogs

Transglutaminase 1 (TGM1) plays a central role in the cross-linking of proteins during the cornification of the epidermis in mammals. Mutations of human TGM1 lead to TGM1-nonsyndromic epidermal differentiation disorder (TGM1-nEDD), and the deletion of Tgm1 is lethal in the mouse. Here, we investigated whether the role of TGM1 is conserved in the western clawed frog ( Xenopus tropicalis ), a model species for amphibians. We found that a tgm1 gene is conserved in the three main clades of amphibians, namely anurans, caudates and caecilians. The amino acid residues implicated in catalytic activity and palmitoylation-dependent membrane anchorage are conserved in amphibian Tgm1 proteins. tgm1 is expressed besides multiple homologs of tgm3 in the skin of X. tropicalis . CRISPR-Cas9-mediated disruption of tgm1 abolished the cell membrane-associated TGM activity in the epidermis of X. tropicalis . In contrast to mosaic mutants and heterozygous mutants of tgm1 , compound heterozygous frogs carrying two predicted loss-of-function alleles of tgm1 died around metamorphosis. In conclusion, Tgm1 is catalytically active at keratinocyte cell membranes during epidermal cornification of X. tropicalis . The conservation of this role of Tgm1 in frogs and mammals supports the hypothesis that Tgm1-dependent cornification evolved in a common ancestor of amphibians and amniotes.