A novel big defensin identified in horseshoe crab hemocytes: Isolation, amino acid sequence, and antibacterial activity

Tetsu Saito, Shun Ichiro Kawabata, Takeshi Shigenaga, Yoko Takayenoki, Junko Cho, Hiroshi Nakajima, Michimasa Hirata, Sadaaki Iwanaga

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Abstract

Hemocytes of the horseshoe crab (limulus) contain a family of arthropodous peptide antibiotics, termed the tachyplesin family, and antibacterial protein, called anti-LPS factor, of which the former is located in the small (S) granules and the latter in the large (L) granules of the hemocytes. In our ongoing studies on granular components, we have identified here a novel defensin-like substance present in both L- and S-granules. This substance strongly inhibits the growth of Gram-negative and -positive bacteria, and fungi, such as Candida albicans. The isolated substance, tentatively termed "big defensin," consists of 79 amino acid residues, of which the COOH-terminal 37 residues have a sequence similar to those of mammalian neutrophil-derived defensins, especially rat defensin. Characterization of the disulfide motif in big defensin indicated that the disulfide array is identical to that of β-defensins from bovine neutrophils. One clear structural difference is that the limulus hemocyte-derived big defensin has an extension of the NH2-terminal hydrophobic sequence with 35 amino acid residues followed by the COOH-terminal cationic defensin portion. This amphipathic nature of big defensin seems likely to be associated with its potent antibacterial activity. Furthermore, antibacterial activities of the NH2-terminal hydrophobic region and the COOH-terminal defensin portion separated by tryptic digestion are significantly different: the former displays a more potent activity against Gram-positive bacteria, whereas the latter is more potent against Gram-negative bacteria. Big defensin, therefore, may prove to represent a new class of defensin family possessing two functional domains with different antimicrobial activities.

Original languageEnglish
Pages (from-to)1131-1137
Number of pages7
JournalJournal of biochemistry
Volume117
Issue number5
DOIs
Publication statusPublished - May 1995

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All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology

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