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)