Deimination of Human Hornerin Enhances its Processing by Calpain-1 and its Cross-Linking by Transglutaminases

Abstract

Hornerin (HRNR) shares numerous features with filaggrin, a key contributor to the epidermal barrier functions. The two proteins display a related structural organization, are expressed by the granular keratinocytes as a large precursor processed by proteolysis, and are cross-linked to the cornified cell envelopes. Two main steps in the metabolism of filaggrin are its deimination and calpain-1 cleavage. Here, using ion-exchange chromatography and two-dimensional gel electrophoresis of human epidermis extracts, we determined that HRNR is deiminated invivo. Accordingly, cornified envelopes, purified from plantar and abdominal human skin, were shown to contain deiminated proteins. A recombinant form of HRNR (HRNRHis) deiminated invitro was shown to be a better substrate for transglutaminases 1 and 3 than the unmodified form. Our data also indicated that calpain-1 may be involved in the proteolytic processing of HRNR, because calpain-1 was co-located with HRNR in the cytoplasm of granular keratinocytes. Using Western blotting and mass spectrometry analysis, HRNRHis was shown to be cleaved by calpain-1 invitro, its deimination enhancing its proteolysis. In HRNR full sequence, four calpain-1 cleavage sites were identified. Altogether, these data allowed a new role to be deciphered for deimination during cornification and provided further characterization of HRNR metabolism.