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Effects of melittin on molecular dynamics and Ca-ATPase activity in sarcoplasmic reticulum membranes: time-resolved
J Voss1, W Birmachu, D M Hussey
1Department of Biochemistry, University of Minnesota Medical School, Minneapolis 55455.
Abstract:
We have studied the effect of melittin, a basic membrane-binding peptide, on Ca-ATPase activity and on protein and lipid dynamics in skeletal sarcoplasmic reticulum (SR), using time-resolved phosphorescence and fluorescence spectroscopy. Melittin completely inhibits Ca-ATPase activity, with half-maximal inhibition at 9 +/- 1 mol of melittin bound to the membrane per mole of ATPase (0.1 mol of melittin per mole of lipid). The time-resolved phosphorescence anisotropy (TPA) decay of the Ca-ATPase labeled with erythrosin isothiocyanate (ERITC) shows that melittin restricts microsecond protein rotational motion. At 25 degrees C in the absence of melittin, the TPA is characterized by three decay components, corresponding to a rapid segmental motion (correlation time phi 1 = 2-3 microseconds), the uniaxial rotation of monomers or dimers (phi 2 = 16-22 microseconds), and the uniaxial rotation of larger oligomers (phi 3 = 90-140 microseconds). The effect of melittin is primarily to decrease the fraction of the more mobile monomer/dimer species (A2) while increasing the fractions of the larger oligomer (A3) and very large aggregates (A infinity). Time-resolved fluorescence anisotropy of the lipid-soluble probe diphenylhexatriene (DPH) shows only a slight increase in the lipid hydrocarbon chain effective order parameter, corresponding to an increase in lipid viscosity that is too small to account for the large decrease in protein mobility or inhibition of Ca-ATPase activity. Thus the inhibitory effect of melittin correlates with its capacity to aggregate the Ca-ATPase and is consistent with previously reported inhibition of this enzyme under conditions that increase protein-protein interactions.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Melittin, a peptide, completely inhibits Ca-ATPase activity by aggregating the enzyme. This peptide restricts protein motion without significantly altering lipid dynamics in skeletal sarcoplasmic reticulum.
Area of Science:
- Biochemistry
- Biophysics
- Membrane protein dynamics
Background:
- Sarcoplasmic reticulum (SR) Ca-ATPase is crucial for muscle contraction.
- Melittin is a basic peptide known to interact with cell membranes.
Purpose of the Study:
- To investigate the effect of melittin on Ca-ATPase activity and protein/lipid dynamics in skeletal SR.
- To elucidate the mechanism of melittin-induced Ca-ATPase inhibition.
Main Methods:
- Time-resolved phosphorescence and fluorescence spectroscopy.
- Labeling Ca-ATPase with erythrosin isothiocyanate (ERITC).
- Using diphenylhexatriene (DPH) as a lipid probe.
Main Results:
- Melittin completely inhibits Ca-ATPase activity.
- Melittin restricts microsecond protein rotational motion by promoting Ca-ATPase aggregation.
- Lipid viscosity changes are insufficient to explain the observed inhibition.
Conclusions:
- Melittin inhibits Ca-ATPase by aggregating the protein, not by altering lipid viscosity.
- The findings correlate melittin's inhibitory effect with increased protein-protein interactions.