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Intermembrane contact affects calcium binding to phospholipid vesicles

R Ekerdt, D Papahadjopoulos

    Proceedings of the National Academy of Sciences of the United States of America
    |April 1, 1982
    PubMed
    Summary
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    Calcium (Ca2+) binding to phosphatidylserine (PtdSer) liposomes shows a specific 1:2 stoichiometry. Membrane contact is crucial for Ca2+ binding affinity, triggering liposome fusion.

    Area of Science:

    • Biochemistry
    • Biophysics
    • Membrane Biology

    Background:

    • Phosphatidylserine (PtdSer) is a key anionic phospholipid in cell membranes.
    • Calcium ions (Ca2+) play critical roles in membrane dynamics, including fusion and aggregation.
    • Understanding Ca2+ binding mechanisms to specific lipids is essential for deciphering membrane-related processes.

    Purpose of the Study:

    • To investigate the binding characteristics of Ca2+ to PtdSer liposomes.
    • To determine the stoichiometry and affinity of Ca2+ binding.
    • To elucidate the role of membrane composition and proximity in Ca2+-PtdSer interactions and their link to membrane fusion.

    Main Methods:

    • Potentiometric titrations were used to analyze Ca2+ binding to large unilamellar vesicles (LUVs) of PtdSer and mixed lipid compositions.

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  • Liposomes composed of PtdSer, phosphatidylethanolamine (PtdEtn), phosphatidylcholine (PtdCho), galactosylcerebroside, and glycolipid GL-4 were utilized.
  • Vesicle aggregation was monitored in conjunction with Ca2+ binding studies.

    • Main Results:

    • Ca2+ binding to PtdSer liposomes is saturable with a 1:2 (Ca2+/PtdSer) stoichiometry.
    • A discontinuity in the binding curve at ~6 x 10(-4) M free [Ca2+] indicates a higher affinity complex formation.
    • Mg2+ can influence Ca2+ binding, and vesicle aggregation correlates with enhanced Ca2+ binding.
    • Incorporation of PtdEtn maintained the 1:2 stoichiometry, while PtdCho abolished the affinity shift (1:4 stoichiometry).
    • Specific glycolipids modulated Ca2+ binding, with GL-4 abolishing the increased binding.

    Conclusions:

    • A specific Ca2+/PtdSer complex with 1:2 stoichiometry forms at membrane contact sites.
    • Close membrane proximity is a prerequisite for the high-affinity Ca2+ binding and subsequent membrane fusion.
    • Membrane composition significantly influences Ca2+ binding affinity and the potential for Ca2+-induced fusion.