Identification of proteolytic cleavage sites in the conversion of profilaggrin to filaggrin in mammalian epidermis

Profilaggrin consists of multiple filaggrin domains joined by linker segments which are removed during proteolytic conversion to filaggrin. Analysis of tryptic peptides of filaggrin defined a 26-residue linker segment when aligned on the amino acid sequence of one repeat unit of mouse profilaggrin deduced from a cDNA sequence (Rothnagel, J. A., Mehrel, T., Idler, W. W., Roop, D. R., and Steinert, P. M. (1987) J. Biol. Chem. 262, 15643-15648). Two types of linker segments were distinguished by their different susceptibility to thermolysin and by the presence of a Phe-Tyr-Pro-Val sequence in only one type. These data led to a model of profilaggrin in which the two types of linker segments alternate along the length of profilaggrin. This model provides a structural basis for the two stages of proteolytic processing seen in vivo. In the first stage intermediates accumulate which have several filaggrin domains still joined by linker segments lacking Phe-Tyr-Pro-Val. In the second stage, the other linker segments are cleaved and mature filaggrin domains are released. Proteolytic activity with specificity consistent with first stage cleavage was partially purified from rat epidermis. Chymostatin inhibited both the in vitro enzymatic activity and the processing of profilaggrin in a cultured rat keratinocyte cell line. The products formed in vitro were 3-5 kDa larger than intermediates produced in vivo, suggesting that the linker segments are cleaved at one end only. This implies the existence of a third protease which completes the removal of the linker segments.