The conserved salt bridge in human α-defensin 5 is required for its precursor processing and proteolytic stability

Mohsen Rajabi, Erik De Leeuw, Marzena Pazgier, Jing Li, Jacek Lubkowski, Wuyuan Lu*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

54 Scopus citations

Abstract

Mammalian α-defensins, expressed primarily in leukocytes and epithelia, play important roles in innate and adaptive immune responses to microbial infection. Six invariant cysteine residues forming three indispensable disulfide bonds and one Gly residue required structurally for an atypical β-bulge are totally conserved in the otherwise diverse sequences of all known mammalian α-defensins. In addition, a pair of oppositely charged residues (Arg/Glu), forming a salt bridge across a protruding loop in the molecule, is highly conserved. To investigate the structural and functional roles of the conserved Arg6-Glu14 salt bridge in human α-defensin 5 (HD5), we chemically prepared HD5 and its precursor proHD5 as well as their corresponding salt bridge-destabilizing analogs E14Q-HD5 and E57Q-proHD5. The Glu-to-Gln mutation, whereas significantly reducing the oxidative folding efficiency of HD5, had no effect on the folding of proHD5. Bovine trypsin productively and correctly processed proHD5 in vitro but spontaneously degraded E57Q-proHD5. Significantly, HD5 was resistant to trypsin treatment, whereas E14Q-HD5 was highly susceptible. Further, degradation of E14Q-HD5 by trypsin was initiated by the cleavage of the Arg13- Gln14 peptide bond in the loop region, a catastrophic proteolytic event resulting directly in quick digestion of the whole defensin molecule. The E14Q mutation did not alter the bactericidal activity of HD5 against Staphylococcus aureus but substantially enhanced the killing of Escherichia coli. By contrast, proHD5 and E57Q-proHD5 were largely inactive against both strains at the concentrations tested. Our results confirm that the primary function of the conserved salt bridge in HD5 is to ensure correct processing of proHD5 and subsequent stabilization of mature α-defensin in vivo.

Original languageEnglish
Pages (from-to)21509-21518
Number of pages10
JournalJournal of Biological Chemistry
Volume283
Issue number31
DOIs
StatePublished - 1 Aug 2008
Externally publishedYes

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