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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)

This family belongs to the: Defensin Superfamily.

References associated with 1.C.45 family:

Broekaert, W.F., B.P.A. Cammue, M.F.C. De Bolle, K. Thevissen, G.W. De Samblanx and R.W. Osborn (1997). Antimicrobial peptides from plants. Crit. Rev. Plant. Sci. 16: 297-323.
Caldwell, J.E., F. Abildgaard, Z. Dzakula, D. Ming, G. Hellekant, and J.L. Markley. (1998). Solution structure of the thermostable sweet-tasting protein brazzein. Nat Struct Biol 5: 427-431. 9628478
Finkina, E.I., E.I. Shramova, A.A. Tagaev, and T.V. Ovchinnikova. (2008). A novel defensin from the lentil Lens culinaris seeds. Biochem. Biophys. Res. Commun. 371: 860-865. 18468512
Garcia-Olmedo, F., A. Molina, J.M. Alamillo and P. Rodriguez-Palenzuela (1998). Plant defense peptides. Biopolymers. 479-491. 10333739
González, R., K. Brokordt, C.B. Cárcamo, T. Coba de la Peña, D. Oyanedel, L. Mercado, and P. Schmitt. (2017). Molecular characterization and protein localization of the antimicrobial peptide big defensin from the scallop Argopecten purpuratus after Vibrio splendidus challenge. Fish Shellfish Immunol 68: 173-179. 28690141
Oomen, R.J., E. Séveno-Carpentier, N. Ricodeau, C. Bournaud, G. Conéjéro, N. Paris, P. Berthomieu, and L. Marquès. (2011). Plant defensin AhPDF1.1 is not secreted in leaves but it accumulates in intracellular compartments. New Phytol 192: 140-150. 21679189
Shafee, T.M., F.T. Lay, M.D. Hulett, and M.A. Anderson. (2016). The Defensins Consist of Two Independent, Convergent Protein Superfamilies. Mol Biol Evol 33: 2345-2356. 27297472
Shafee, T.M., F.T. Lay, T.K. Phan, M.A. Anderson, and M.D. Hulett. (2017). Convergent evolution of defensin sequence, structure and function. Cell Mol Life Sci 74: 663-682. 27557668
Yount, N.Y. and M.R. Yeaman. (2004). Multidimensional signatures in antimicrobial peptides. Proc. Natl. Acad. Sci. USA 101: 7363-7368. 15118082
Zhao, Q., Y.K. Chae, and J.L. Markley. (2002). NMR solution structure of ATTp, an Arabidopsis thaliana trypsin inhibitor. Biochemistry 41: 12284-12296. 12369816