1.C.45 The Plant Defensin (Plant Defensin) Family
Plant defensins form ion channels capable of transporting H+, Ca2+ and K+. However a concentration as great as 10x above the toxic concentration may be required to demonstrate increased permeability. The mature peptides are about 45-54aas in length. They possess eight disulfide linked cysteines. They were initially called γ-thionins, but x-ray analyses revealed that thionins and plant defensins are structurally unrelated. Defensins are typified by a triple stranded antiparallel β-sheet plus one α-helix. The two cysteines in the CXXXC segment of the α-helix are connected to the two cysteines in the CXC segment of the C-terminal β-strand. They resemble insect defensins in fold except for the amino terminal β-strand of the plant defensins. The conserved residues include the 8 Cs, 2 Gs (at positions 13 and 34), an aromatic residue (at position 11), and an E (at position 29). Multiple plant defensin genes are found in a single plant genome.
Shafee et al. 2016 have suggested that defensins and small defensin-like proteins fall into two superfamilies, which they call the cis-defensins (broadly distributed in living organisms) and the trans-defensins (narrowly distrubuted). They suggest that these two groups of proteins converged to show similar sequences, secondary and tertiary structures, and disulfide connectivities, with overlapping organismal sources and functions, in spite of their independent origins. The functions of these short proteins vary tremendously including pore formation, bacterial and fungal toxicity, lipid targeting, toxic receptor and channel interactions, fertilization, protease inhibiton and stress adaptation. However, as noted by the authors, alternative pathways involving divergent evolution from a common evolutionary source could have also occurred although they consider this possibility less likely (Shafee et al. 2017).
The generalized transport reaction catalyzed by plant defensins is:
small molecules (in) ⇌ small molecules (out)
g-2 purothionin, the antifungal lentil seed defensin, Lc-def (47aas plus of 27aa leader peptide) (Finkina et al., 2008)
Precursor of Lc-def of Lens culinaris (B3F051)
The antifungal lentil seed defensin, Lc-def (47aas plus of 27aa leader peptide) (Finkina et al., 2008)
Defensin Lc-def of Lens culinaris
γ-thionin or defensin J1-2 of 74 aas
Thionin of Capsicum annuum (bell pepper)
Plant vacuolar defensin, Pdf1.1, of 80 aas. It plays an antifungal role, and is involved in abiotic stress tolerance as well as inhibition of root growth (Oomen et al. 2011).
Pdf1.1 of Arabidopsis thaliana
Knol1 domain-containing protein of 81 aas and 1 N-terminal TMS.
Knot1 of Setaria italica (Foxtail millet) (Panicum italicum)
Defensin-like protein 106 of 106 aas
Defensin of Arabidopsis thaliana
Defensin-like protein 107 of 81 aas
Defensin of Arabidopsis thaliana
Defensin-like protein of 74 aas
Defensin of Ipomoea trifida (Morning glory)
Nasonin-1 of 56 aas and 1 N-terminal TMS; has antibacterial activity. The 3-D structure is known (2KOZ).
Nasonin-1 of Nasonia vitripennis (Parasitic wasp)
Uncharacterized protein of 73 aas and 1 N-terminal TMS.
UP of Arabidopsis lyrata subsp. lyrata (Lyre-leaved rock-cress)
Defensin-like protein 195 of 89 aas and 1 N-terminal TMS, It is a serine (trypsin-like) protease inhibitor, and its NMR sturcture is available (Zhao et al. 2002).
UP of Arabidopsis thaliana
Defensin-like protein 195, brazzein, of 54 aas; It is a taste-modifying protein, sweet-tasting. It is 2000 sweeter than sucrose on a molar basis (Caldwell et al. 1998). It has a pH-specific antimicrobial activity against bacteria (B. subtilis, E. coli and S. aureus) and the fungus C. albicans (Yount and Yeaman 2004).
Brazzein of Pentadiplandra brazzeana
Big defensin-1, BD1, of 124 aas and 2 TMSs, an antimicrobial peptide (González et al. 2017).
BD1 of Argopecten purpuratus (Chilean northern scallop)
Big defensin, BD, of 117 aas and 2 TMSs
BD of Tachypleus tridentatus (Japanese horseshoe crab)
Big defensin isoform XI of 169 aas and 2 TMSs.
BD of Crassostrea gigas
Big defensin of 111 aas and 2 TMSs
BD of Anadara broughtonii (Blood clam) (Scapharca broughtonii)
Big defensin of 162 aas and 2 TMSs
BD of Lingula unguis