On the coordination of La3+ by phosphatidylserine

In a recent study by Bentz, J., D. Alford, J. Cohen, and N. Düzgünes (1988. Biophys. J. 53:593-607), La3+ was found to be more effective than Ca2+ in causing nonleaky fusion of phosphatidylserine vesicles. It was proposed that this difference in fusion efficiency may be due, in part, to a difference in coordination of the two cations. That is, Ca2+ was presumed to bind to the lipid phosphate, whereas La3+ was proposed to be coordinated by the serine carboxylate and amine. 31P and 13C NMR results presented here demonstrate that the lanthanides, Tb3+ and La3+, are coordinated by the phosphodiester and carboxylate moieties of phosphatidylserine. Tb3+-Phosphatidylserine optical experiments suggest that the serine amine does not coordinate the lanthanide below pH 10, at least not while the membrane has a net negative surface charge. Although these observations disagree with the structural details proposed by Bentz et al. (1988), they are not in conflict with their general fusion mechanism. The work presented here also demonstrates that La3+ affects the inner surface phosphodiesters differently than those on the outer surface of phosphatidylserine vesicles. The vesicles studied are of an intermediate size, having diameters on the order of 150-200 nm. The cation appears to have a more immediate effect on the packing of the crowded headgroups on the inner surface. Higher levels of bound La3+ on the outer surface may be required to induce the same changes in headgroup conformation.