Response: Extracellular localization of platelet SNARE proteins

We thank Hirai et al for their comments and regret not having cited in our recent article the interesting work previously performed on epimorphin/syntaxin-2, including its localization to extracellular cell surfaces. We agree that the role of syntaxin-2 on the platelet surface is unclear. Previous work demonstrating that extracellular epimorphin/syntaxin-2 acts as a morphogen suggests important alternative and/or additional roles (eg, signaling, adhesion) to those outlined in “Discussion” in our article.¹  As indicated in the correspondence of Hirai et al, we suggested that extracellular SNARE proteins may mediate fusion of extracellular membranes. To serve this function, syntaxin-2 would be expected to interact with cognate SNARE proteins, forming a complex analogous to that which directs fusion of intracellular membranes. Expression of SNAP-23 on the extracellular surface of the platelet plasma membrane is described in our study, which also demonstrates that extracellular SNAP-23 binds syntaxin-2. Since the publication of these experiments, we have identified VAMP-8 on the extracellular surface of platelets (N.R., R.F., unpublished observations, April 2007). This result is intriguing in light of the studies of Ren et al²  that indicate that VAMP-8 is the primary vSNARE mediating platelet granule release. Thus, vSNARE and tSNARE isoforms required for forming a fusion-competent complex are found on the extracellular surface of the platelet. The fact that HeLa cells engineered to express extracellular SNARE proteins fuse to one another indicates that extracellular SNARE proteins are capable of mediating cell-cell fusion.^3,4  Whether SNARE proteins actually form a complex on the extracellular surface of platelets and are capable of mediating platelet-platelet fusion will require further evaluation.