Background: Infections caused by Candida albicans, a common fungal pathogen of humans, are increasing in incidence, necessitating development of new therapeutic drugs. Secreted aspartic proteinase (SAP) activity is considered an important virulence factor in these infections and might offer a suitable target for drug design. Amongst the various SAP isozymes, the SAP2 gene product is the major form expressed in a number of C. albicans strains.
Results: The three-dimensional structures of SAP2 complexed with the tight-binding inhibitor A70450 (a synthetic hexapeptide analogue) and with the general aspartic proteinase inhibitor pepstatin A (a microbial natural product) have been determined to 2.1 A and 3.0 A resolution, respectively. Although the protein structure retains the main features of a typical aspartic proteinase, it also shows some significant differences, due mainly to several sequence insertions and deletions (as revealed by homology modelling), that alter the shape of the binding cleft. There is also considerable variation in the C-terminal structural domain.
Conclusions: The differences in side chains, and in the conformations adopted by the two inhibitors, particularly at their P4, P3 and P'2 positions (using standard notation for protease-inhibitor residues), allows the A70450 structure to complement, more accurately, that of the substrate-binding site of SAP2. Some differences in the binding clefts of other SAP isoenzymes may be deduced from the SAP2 structure.