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Melittin induces HII phase formation in cardiolipin model membranes
A M Batenburg1, J C Hibbeln, A J Verkleij
1Department of Biochemistry, State University of Utrecht, The Netherlands.
Abstract:
The interaction of melittin with bovine heart cardiolipin model membranes was investigated via binding assays, 31P-NMR, freeze-fracture electron microscopy, small angle X-ray diffraction and fluorescence based fusion assays. A strong binding (Kd less than 10(-7) M) appeared to be accompanied by the formation of large structures, resulting from a fusion process of extremely fast initial rate. As the melittin content is increased, bilayer structure is gradually lost and from a cardiolipin to melittin ratio of about 6 the lipid starts to organize itself in an hexagonal HII phase. At lower temperatures (T less than 40 degrees C) the coexistence of another structure is observed, characterized by a broad isotropic 31P-NMR signal and giving rise to sharp X-ray reflections, most probably a cubic phase, as suggested also be freeze-fracture images, showing orderly stacked particles. The results are discussed in relation to contrasting observations on the structural changes induced by melittin in the zwitterionic phospholipid system of dipalmitoylphosphatidylcholine (Dufourcq. J. et al. (1986) Biochim. Biophys. Acta 859, 33-48). The biological relevance of the observations with respect to the process of protein insertion into membranes is indicated.
Insights
Melittin strongly binds to cardiolipin membranes, inducing rapid fusion and structural changes. At higher melittin concentrations, lipids form hexagonal HII and cubic phases, impacting membrane organization.
Area of Science:
- Biochemistry
- Biophysics
- Membrane Biology
Background:
- Cardiolipin is a key anionic phospholipid in mitochondrial membranes.
- Melittin, a peptide toxin, interacts with lipid bilayers, altering their structure and function.
Purpose of the Study:
- To investigate the structural consequences of melittin interaction with cardiolipin model membranes.
- To elucidate the mechanism of membrane fusion induced by melittin.
Main Methods:
- Binding assays
- 31P-Nuclear Magnetic Resonance (NMR)
- Freeze-fracture electron microscopy
- Small-angle X-ray diffraction
- Fluorescence-based fusion assays
Main Results:
- Strong melittin binding (Kd < 10(-7) M) led to rapid membrane fusion and large structure formation.
- Increased melittin concentration disrupted bilayer structure, promoting hexagonal HII phase formation at a cardiolipin to melittin ratio of ~6.
- At temperatures below 40°C, coexistence of hexagonal HII and a cubic phase was observed, evidenced by NMR and X-ray diffraction.
Conclusions:
- Melittin induces significant structural rearrangements in cardiolipin membranes, including fusion and phase transitions.
- The observed structural changes provide insights into protein-membrane interactions and potential mechanisms of membrane insertion.