SKALP ELAFIN - AN ELASTASE INHIBITOR FROM CULTURED HUMAN KERATINOCYTES - PURIFICATION, CDNA SEQUENCE, AND EVIDENCE FOR TRANSGLUTAMINASE CROSS-LINKING/

SKALP/Elafin is a proteinase inhibitor found in psoriatic epidermis as a short polypeptide of 6 kDa. Here we present evidence that this prot ein is synthesized as a larger precursor molecule with distinct biolog ical features. Purification and NH2-terminal sequencing of SKALP/elafi n from cultured human keratinocytes and the cloning of its cDNA reveal ed the existence of a mature protein, which upon cleavage of a hydroph obic signal sequence of 22 amino acids has a calculated molecular mass of 9.9 kDa (95 amino acids). In addition to the known proteinase inhi bitor domain, the mature protein contains a domain with 4 repeats whic h are homologous to putative transglutaminase substrate motifs. We wer e able to demonstrate on Western blots that immunoreactive SKALP is pr esent in high molecular weight proteins extracted from psoriatic skin. This suggests that SKALP is covalently attached to epidermal proteins . In addition it was found that both the complete SKALP molecule and a synthetic peptide of the NH2-terminal portion of SKALP could be used as a transglutaminase substrate. We therefore speculate that SKALP/ela fin, secreted by epidermal keratinocytes in inflamed skin, exists both as a free 6-kDa form and as an immobilized 9.9-kDa form covalently at tached to the cornified envelopes by transglutaminase cross-linking.