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Biophys J, August 1999, p. 829-841, Vol. 77, No. 2

Ultrastructural Characterization of Peptide-Induced Membrane Fusion and Peptide Self-Assembly in the Lipid Bilayer

Anne S. Ulrich,* Willem Tichelaar,# Günter Förster,§ Olaf Zschörnig, Sevil Weinkauf,# and Helmut W. Meyer#

 *Institut für Molekularbiologie, Friedrich-Schiller-Universität Jena, 07745 Jena;  #Institut für Ultrastrukturforschung des Klinikums der Friedrich-Schiller-Universität Jena, 07743 Jena;  §Institut für Physikalische Chemie, Martin-Luther-Universität Halle, 06108 Halle; and  Institut für Medizinische Physik und Biophysik, 04103 Leipzig

The peptide sequence B18, derived from the membrane-associated sea urchin sperm protein bindin, triggers fusion between lipid vesicles. It exhibits many similarities to viral fusion peptides and may have a corresponding function in fertilization. The lipid-peptide and peptide-peptide interactions of B18 are investigated here at the ultrastructural level by electron microscopy and x-ray diffraction. The histidine-rich peptide is shown to self-associate into two distinctly different supramolecular structures, depending on the presence of Zn2+, which controls its fusogenic activity. In aqueous buffer the peptide per se assembles into beta -sheet amyloid fibrils, whereas in the presence of Zn2+ it forms smooth globular clusters. When B18 per se is added to uncharged large unilamellar vesicles, they become visibly disrupted by the fibrils, but no genuine fusion is observed. Only in the presence of Zn2+ does the peptide induce extensive fusion of vesicles, which is evident from their dramatic increase in size. Besides these morphological changes, we observed distinct fibrillar and particulate structures in the bilayer, which are attributed to B18 in either of its two self-assembled forms. We conclude that membrane fusion involves an alpha -helical peptide conformation, which can oligomerize further in the membrane. The role of Zn2+ is to promote this local helical structure in B18 and to prevent its inactivation as beta -sheet fibrils.

Biophys J, August 1999, p. 829-841, Vol. 77, No. 2
© 1999 by the Biophysical Society   0006-3495/99/08/829/13  $2.00


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