Molecular cloning of the cDNA and gene for an elastinolytic aspartic proteinase from Aspergillus fumigatus and evidence of its secretion by the fungus during invasion of the host lung.

Hydrolysis of structural proteins in the lung by extracellular proteinases secreted by Aspergillus fumigatus is thought to play a significant role in invasive aspergillosis. This fungus was found previously to secrete an elastinolytic serine proteinase and a metalloproteinase. We report that A. fumigatus also secretes an aspartic proteinase (aspergillopepsin F) that can catalyze hydrolysis of the major structural proteins of basement membrane, elastin, collagen, and laminin. The pH optimum for the enzymatic activity was 5.0 with elastin-Congo red as the substrate, and the activity was not significantly inhibited by pepstatin A, diazoacetyl norleucine methylester, and 1,2-epoxy-3-(p-nitrophenoxy) propane. The cDNA and gene encoding this aspartic proteinase were cloned and sequenced. The open reading frame, interrupted by three introns, would encode a protein of 393 amino acids composed of a putative 21-amino-acid signal peptide and a 49-amino-acid propeptide preceding the 323-amino-acid mature protein. The amino acid sequence of A. fumigatus aspartic proteinase has 70, 66, and 67% homology to the sequences of those from Aspergillus oryzae, Aspergillus awamori, and Aspergillus saitoi, respectively. The active-site motif (DTG) and the catalytic aspartic residues characteristic of aspartic proteinases are found in the presently described enzyme, indicating that it belongs to a family of aspartic proteinases. Polyclonal antibodies were produced in rabbits against both the mature and precursor forms of the aspartic proteinase expressed in Escherichia coli. Immunoblotting with both antibodies detected a 39-kDa mature enzyme in the culture supernatant of A. fumigatus. The aspartic proteinase activity was inhibited by the antibodies, suggesting that the aspartic proteinase in the culture supernatant corresponds to the product of the cloned gene. Immunogold electron microscopy showed that the aspartic proteinase was secreted by A. fumigatus invading neutropenic mouse lung and its secretion was directed toward the germ tubes of penetrating hyphae.