Arthropod defensin
Structure of insect defensin A. [1]
Identifiers
Symbol	Defensin_2
Pfam	PF01097
InterPro	IPR001542
PROSITE	PDOC00356
SCOP2	1ica / SCOPe / SUPFAM
TCDB	1.C.47
OPM superfamily	58
OPM protein	1l4v
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Arthropod defensins are a family defensin proteins found in mollusks, insects, and arachnids. These cysteine-rich antibacterial peptides are primarily active against Gram-positive bacteria and fungi in vitro. ^{[2] [3] [4] [5] [6]} However Drosophila fruit flies mutant for the fly defensin were more susceptible to infection by the Gram-negative bacteria Providencia burhodogranariea, and resisted infection against Gram-positive bacteria like wild-type flies. ^[7] It remains to be seen how in vitro activity relates to in vivo function. Mutants for the defensin-like antimicrobial peptide Drosomycin were more susceptible to fungi, validating a role for defensin-like peptides in anti-fungal defence. ^[7]

Arthropod defensin peptides range in length from 38 to 51 amino acids. There are six conserved cysteines all involved in intrachain disulfide bonds. Studies have shown that the cysteine-bridge disulfide bonds are not required for antimicrobial activity, ^[8] similar to findings in mammalian defensins. ^[9] Furthermore, it was also shown that the N-terminal helix region in arthropod or insect defensins is also not required for antimicrobial activity of these peptides. ^[8]

A schematic representation of peptides from the arthropod defensin family is shown below.

           +----------------------------+
           |                            |
         xxCxxxxxxxxxxxxxxCxxxCxxxxxxxxxCxxxxxCxCxx
                          |   |               | |
                          +---|---------------+ |
                              +-----------------+

'C': conserved cysteine involved in a disulfide bond.

Sequence similarities have been reported between the arthropod defensins and mammalian defensins. ^{[10] [2]} However it appears that defensins of vertebrates, arthropods, plants, and fungi arose independently. ^[11] This is supported by 3D structural differences between arthropod defensins and vertebrate beta defensins. ^[12] However structural similarities exist between these defensins, notably in two structural motifs termed "C6" and "C8". This has prompted a higher "cis-" or "tras-" defensin classification system wherein the structural relationships of the shared motifs is used to delineate defensin similarities. ^[11]

Defensins of mammals display anti-cancer activities in vitro, ^[13] and down-regulation of human beta-defensin 1 is associated with increased risk of prostate cancer and clear-cell carcinomas. ^[14] The first in vivo anti-cancer functions for defensin came from Drosophila studies, which showed that the Drosophila defensin attacks tumor cells, and that flies lacking defensin had greater tumor growth in a cancer disease model. ^{[15] [16]}

Overactive immune signalling is also implicated in age-associated neurodegeneration, ^[17] and overexpression of defensin leads to increased degradation of brain tissue. ^[18]

• This article incorporates text from the public domain Pfam and InterPro: IPR001542
• PDOC00356 - Arthropod defensins in PROSITE (=source of figure)

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