Evidence for degradation of gastrointestinal mucin by Candida albicans secretory aspartyl proteinase

A zone of extracellular digestion of the mucin layer around Candida albicans blastoconidia was observed by transmission electron microscopy in the jejunum of mice inoculated intragastrically (G. T. Cole, K. R. Seshan, L. M. Pope, and R. J. Yancey, J. Med. Vet. Mycol. 26:173-185, 1988). This observation prompted the hypothesis that a putative mucinolytic enzyme(s) may contribute to the virulence of C. albicans by facilitating penetration of the mucus barrier and subsequent adherence to and invasion of epithelial cells. Mucinolytic activity was observed as zones of clearing around colonies of C. albicans LAM-1 grown on agarose containing yeast nitrogen base, glucose, and hog gastric mucin. In addition, concentrated culture filtrate obtained after growth for 24 h in yeast nitrogen base, supplemented with glucose and mucin as the sole nitrogen source, contained proteolytic activity against biotin-labelled mucin which was inhibited by pepstatin A. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the culture filtrate revealed two components of 42 and 45 kDa, with pIs of 4.1 and 5.3, respectively. A zymogram showed that mucin was degraded only by the 42-kDa component, which was also recognized by immunoblotting with an anti-secretory aspartyl proteinase (anti-Sap) 2p monoclonal antibody. The N-terminal sequence of the first 20 amino acids matched that reported for Sap2p. These results demonstrate that Sap2p is responsible for proteolysis of mucin by C. albicans in vitro and may be involved as a virulence factor in the breakdown of mucus and penetration of the mucin barrier by C. albicans.