The Journal of General Physiology
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Published online
doi:10.1085/jgp.200609685
The Journal of General Physiology, Vol. 129, No. 3, 233-244
The Rockefeller University Press, 0022-1295 $30.00
© Yang et al.
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ARTICLE

 Phosphomimetic Mutation of Ser-187 of SNAP-25 Increases both Syntaxin Binding and Highly Ca2+-sensitive Exocytosis



Yan Yang1, Tim J. Craig4, Xiaohui Chen1,2, Leonora F. Ciufo4, Masami Takahashi5, Alan Morgan4, and Kevin D. Gillis1,2,3

1 Dalton Cardiovascular Research Center, 2 Department of Biological Engineering, 3 Department of Medical Pharmacology and Physiology, University of Missouri, Columbia, MO 65211
4 The Physiological Laboratory, School of Biomedical Sciences, University of Liverpool, UK
5 Department of Biochemistry, Kitasato University School of Medicine, Kanagawa, Japan

Correspondence to Kevin D. Gillis: gillisk{at}missouri.edu; or Alan Morgan: amorgan{at}liv.ac.uk

The phosphorylation targets that mediate the enhancement of exocytosis by PKC are unknown. PKC phosporylates the SNARE protein SNAP-25 at Ser-187. We expressed mutants of SNAP-25 using the Semliki Forest Virus system in bovine adrenal chromaffin cells and then directly measured the Ca2+ dependence of exocytosis using photorelease of caged Ca2+ together with patch-clamp capacitance measurements. A flash of UV light used to elevate [Ca2+]i to several µM and release the highly Ca2+-sensitive pool (HCSP) of vesicles was followed by a train of depolarizing pulses to elicit exocytosis from the less Ca2+-sensitive readily releasable pool (RRP) of vesicles. Carbon fiber amperometry confirmed that the amount and kinetics of catecholamine release from individual granules were similar for the two phases of exocytosis. Mimicking PKC phosphorylation with expression of the S187E SNAP-25 mutant resulted in an approximately threefold increase in the HCSP, whereas the response to depolarization increased only 1.5-fold. The phosphomimetic S187D mutation resulted in an ~1.5-fold increase in the HCSP but a 30% smaller response to depolarization. In vitro binding assays with recombinant SNARE proteins were performed to examine shifts in protein–protein binding that may promote the highly Ca2+-sensitive state. The S187E mutant exhibited increased binding to syntaxin but decreased Ca2+-independent binding to synaptotagmin I. Mimicking phosphorylation of the putative PKA phosphorylation site of SNAP-25 with the T138E mutation decreased binding to both syntaxin and synaptotagmin I in vitro. Expressing the T138E/ S187E double mutant in chromaffin cells demonstrated that enhancing the size of the HCSP correlates with an increase in SNAP-25 binding to syntaxin in vitro, but not with Ca2+-independent binding of SNAP-25 to synaptotagmin I. Our results support the hypothesis that exocytosis triggered by lower Ca2+ concentrations (from the HCSP) occurs by different molecular mechanisms than exocytosis triggered by higher Ca2+ levels.

Abbreviations used in this paper: GST, glutathione S-transferase; HCSP, highly Ca2+-sensitive pool; RRP, readily releasable pool; SFV, Semliki Forest virus; SRP, slowly releasable pool.


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