Binding of monovalent cations to phosphatidylserine and modulation of Ca2+- and Mg2+-induced vesicle fusion

Shlomo Nir*, Nejat Düzgüneş, Joe Bentz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

50 Scopus citations

Abstract

The effect of several monovalent cations on the Ca2+-induced aggregation and fusion of sonicated phosphatidylserine (PS) vesicles is studied by monitoring the mixing of internal compartments of the fusing vesicles using the Tb/dipicolinic acid assay. The dissociation of the fluorescent Tb-dipicolinate complex which accompanies Ca2+-induced vesicle fusion is determined directly and is due to leakage of contents and entry of medium into vesicles. PS vesicles do not fuse when the medium contains only monovalent cations (at pH 7.4), regardless of the cation concentration or whether there is aggregation of the vesicles. A mass-action kinetic analysis of the data provides estimates for the rate of aggregation, C11, and for the rate of fusion per se, f11. Values of f11 increase dramatically with reduction in monovalent cation concentration and are primarily determined by binding ratios of Ca2+ or Mg2+ per PS. With 300 mM of monovalent cations, the fusion per se is essentially rate-limiting to the overall fusion process and values of f11 are significantly larger with the monovalent cations which bind the least, i.e., according to the sequence tetramethylammonium > K+ > Na+ > Li+. With monovalent cations in concentrations of 100 mM or less, the aggregation is rate-limiting to the fusion and the overall initial fusion rates are determined by an interplay between aggregation and fusion rates. Under conditions of fast aggregation, the Ca2+-induced fusion of small PS vesicles can occur within milliseconds or less.

Original languageEnglish
Pages (from-to)160-172
Number of pages13
JournalBiochimica et Biophysica Acta - Biomembranes
Volume735
Issue number1
DOIs
StatePublished - 26 Oct 1983
Externally publishedYes

Keywords

  • Ca
  • Cation binding
  • Phosphatidylserine
  • Vesicle fusion

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