Linkage between phosphorylated cystatin α and filaggrin by epidermal transglutaminase as a model of cornified envelope and inhibition of cathepsin L activity by cornified envelope and the conjugated cystatin α

Lysine-rich phosphorylated cystatin alpha (P-cystatin alpha) from newborn rat epidermis is a good substrate for epidermal transglutaminase (TGase) and also one of the component proteins of cornified envelope in the stratum corneum. Since the filaggrin linker segment peptide was efficiently conjugated with P-cystatin alpha and was mediated by epidermal TGase in the presence of Ca2+ ions, filaggrin is a candidate for the glutamine-rich linkage protein to conjugate with lysine-rich P-cystatin alpha. A conjugated protein was found by epidermal TGase in the activated condition with Ca2+ ions and dithiothreitol. In contrast, the conjugated protein was not formed under chelated conditions with EDTA. The conjugated protein reacted positively with anti-P-cystatin alpha polyclonal antibody (PoAb). The conjugated protein and purified cornified envelope showed an inhibitory effect against papain and cathepsin L, but cathepsin B and H were not inhibited by these P-cystatin alpha conjugates. Although the component protein, P-cystatin alpha itself, inhibited cathepsin H strongly, these conjugated proteins inhibited specifically the cathepsin L family. The amino acid composition of cornified envelope protein and the conjugated protein of P-cystatin alpha and filaggrin linker segment peptide was not completely the same. The conjugated protein of P-cystatin alpha and filaggrin linker segment peptide showed the same inhibitory properties against cysteine proteinases as the cornified envelope. These findings suggest that the linkage protein between P-cystatin alpha and filaggrin linker segment peptide may be considered a model of cornified envelope, although skin cornified envelope may be conjugated with some additional proteins.