The role of the C terminus of the SNARE protein SNAP-25 in fusion pore opening and a model for fusion pore mechanics

doi:10.1073/pnas.0805377105

The role of the C terminus of the SNARE protein SNAP-25 in fusion pore opening and a model for fusion pore mechanics

*School of Applied and Engineering Physics, 212 Clark Hall, Cornell University, Ithaca, NY 14853; and
^§Department of Membrane Biophysics, Max Planck Institute for Biophysical Chemistry, D-37077 Göttingen, Germany

Edited by Axel T. Brunger, Stanford University, Stanford, CA, and approved September 4, 2008 (received for review June 2, 2008)

Abstract

Formation of a fusion pore between a vesicle and its target membrane is thought to involve the so-called SNARE protein complex. However, there is no mechanistic model explaining how the fusion pore is opened by conformational changes in the SNARE complex. It has been suggested that C-terminal zipping triggers fusion pore opening. A SNAP-25 mutant named SNAP-25Δ9 (lacking the last nine C-terminal residues) should lead to a less-tight C-terminal zipping. Single exocytotic events in chromaffin cells expressing this mutant were characterized by carbon fiber amperometry and cell-attached patch capacitance measurements. Cells expressing SNAP-25Δ9 displayed smaller amperometric “foot-current” currents, reduced fusion pore conductances, and lower fusion pore expansion rates. We propose that SNARE/lipid complexes form proteolipid fusion pores. Fusion pores involving the SNAP-25Δ9 mutant will be less tightly zipped and may lead to a longer fusion pore structure, consistent with the observed decrease of fusion pore conductance.

Footnotes

^¶To whom correspondence should be addressed. E-mail: ml95{at}cornell.edu
Author contributions: M.L. designed research; Q.F., K.B., L.-W.G., and I.H. performed research; J.B.S. contributed new reagents/analytic tools; Q.F., K.B., L.-W.G., and I.H. analyzed data; and Q.F. and M.L. wrote the paper.
↵^†Present address: Department of Biological Sciences, University of Illinois, Chicago, IL 60607.
↵^‡Present address: Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada V6T 1Z3.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.