Divergent solid-phase synthesis and candidacidal activity of MUC7 D1, a 51-residue histidine-rich N-terminal domain of human salivary mucin MUC7.

Abstract

Domain 1 of the low-molecular-weight human salivary mucin, designated MUC7 D1, spans the 51 N-terminal amino acid residues. This domain contains a 15-residue basic histidine-rich subdomain (R3-Q17) which has 53% sequence similarity to histatin 5 (Hsn-5), a salivary molecule known to exert potent in vitro cidal activity against Candida albicans and many other medically important fungi. The MUC7 D1-15mer and its derivatives have previously been synthesized in our laboratory and their candidacidal activities have been found to be inferior to that of Hsn-5. We were therefore intrigued to explore the candidacidal potency of the full-length MUC7 D1 (51-mer). Linear solid-phase synthesis of this domain has been accomplished following standard Fmoc chemistry. The problems of partial coupling, owing to the peptide chain length, at several stages of the solid-phase step-by-step synthesis were circumvented either by double-coupling techniques or efficient coupling procedures. The MUC7 D1 peptide was purified to homogeneity by conventional reverse-phase HPLC using two columns connected in series. Secondary structure of the purified peptide was assessed by circular dichroism (CD) spectroscopy in phosphate buffer and trifluoroethanol and compared to that of MUC7 D1-15mer and Hsn-5. The MUC7 D1 candidacidal activity was assessed against azole-sensitive and azole-resistant C. albicans strains and was found, unlike that of the MUC7 D1-15mer, to be comparable with that of Hsn-5, indicating that in addition to Hsn-5, MUC7 D1 could provide an attractive alternative to the classical antifungal agents. The candidacidal potency of MUC7 D1, like that of MUC7 D1-15mer, and of Hsn-5, appears to be largely dependent on peptide charge, irrespective of alpha-helical structure.