Basic residues in azurocidin/HBP contribute to both heparin binding and antimicrobial activity

Azurocidin/CAP37/HBP is an antimicrobial and chemotactic protein that is part of the innate defenses of human neutrophils. In addition, azurocidin is an inactive serine protease homolog with binding sites for diverse ligands including heparin and the bovine pancreatic trypsin inhibitor (BPTI). The structure of the protein reveals a highly cationic domain concentrated on one side of the molecule and responsible for its strong polarity. To investigate the role of this highly basic region, we produced three recombinant azurocidin mutant proteins that were altered in either one or both of two clusters of 4 basic residues located symmetrically on each side of a central cleft in the cationic domain. Two of the mutant proteins (Loop 3: R5Q, K6Q, R8Q, and R10Q; Loop 4: R61Q, R62Q, R63Q, and R65Q) exhibited little or no change in heparin and BPTI binding or in antimicrobial function. In contrast, the Loop 3/Loop 4 mutant (R5Q, K6Q, R8Q, R10Q, R61Q, R62Q, R63Q, and R65Q) in which all 8 basic residues were replaced showed greatly decreased ability to bind heparin and to kill Escherichia coli and Candida albicans. Thus, we report that the 8 basic residues that were altered in the Loop 3/Loop 4 mutant contribute to the ability of the wild-type azurocidin molecule to bind heparin and to kill E. coli and C. albicans. Because BPTI binding was comparable in wild-type and Loop 3/Loop 4 mutant protein, we conclude that the same 8 basic residues are not involved in the binding of BPTI to azurocidin, supporting the notion that the binding site for BPTI is distinct from the site involved in heparin binding and antimicrobial activity. Finally, we show that removal of all 4 positively charged amino acids in the 20-44 azurocidin sequence (DMC1: R23Q,H24S,H32S,R34Q), a region previously thought to contain an antimicrobial domain, does not affect the activity of the protein against E. coli, Streptococcus faecalis, and C. albicans.