Arthropod defensin

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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