Proposal for molecular mechanism of thionins deduced from physico-chemical studies of plant toxins.
We propose a molecular model for phospholipid membrane lysis by the ubiquitous plant toxins called thionins. Membrane lysis constitutes the first major effect exerted by these toxins that initiates a cascade of cytoplasmic events leading to cell death. X-ray crystallography, solution nuclear magnetic resonance (NMR) studies, small angle X-ray scattering and fluorescence spectroscopy provide evidence for the mechanism of membrane lysis. In the crystal structures of two thionins in the family, alpha(1)- and beta-purothionins (MW: approximately 4.8 kDa), a phosphate ion and a glycerol molecule are modeled bound to the protein. (31)P NMR experiments on the desalted toxins confirm phosphate-ion binding in solution. Evidence also comes from phospholipid partition experiments with radiolabeled toxins and with fluorescent phospholipids. This data permit a model of the phospholipid-protein complex to be built. Further, NMR experiments, one-dimensional (1D)- and two-dimensional (2D)-total correlation spectroscopy (TOCSY), carried out on the model compounds glycerol-3-phosphate (G3P) and short chain phospholipids, supported the predicted mode of phospholipid binding. The toxins' high positive charge, which renders them extremely soluble (>300 mg/mL), and the phospholipid-binding specificity suggest the toxin-membrane interaction is mediated by binding to patches of negatively charged phospholipids [phosphatidic acid (PA) or phosphatidyl serine (PS)] and their subsequent withdrawal. The formation of proteolipid complexes causes solubilization of the membrane and its lysis. The model suggests that the oligomerization may play a role in toxin's activation process and provides insight into the structural principles of protein-membrane interactions.
Stec
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B
Department of Chemistry, University of Texas at El Paso, 500 W. University Ave, El Paso, TX 79968, USA. bstec@utep.edu
Markman
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O
Rao
U
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Heffron
G
G
Henderson
S
S
Vernon
L P
LP
Brumfeld
V
V
Teeter
M M
MM
D023181:Antimicrobial Cationic Peptides; D001665:Binding Sites; D002462:Cell Membrane / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D005454:Fluorescence Polarization; D005994:Glycerophosphates / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008958:Models, Molecular; D010743:Phospholipids / Q000737:chemistry* / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D032122:Pyrularia / Q000737:chemistry; D016415:Sequence Alignment; D012995:Solubility; D014118:Toxins, Biological / Q000737:chemistry* / Q000378:metabolism
D023181:Antimicrobial Cationic Peptides; D005994:Glycerophosphates; D010743:Phospholipids; D010940:Plant Proteins; D014118:Toxins, Biological; C036092:hordothionin protein, Hordeum vulgare; C016085:purothionin; C029620:alpha-glycerophosphoric acid