The SNARE Motif Is Essential for the Formation of Syntaxin Clusters in the Plasma Membrane

In the plasma membrane, syntaxin 1 and syntaxin 4 clusters define sites at which secretory granules and caveolae fuse, respectively. It is widely believed that lipid phases are mandatory for cluster formation, as cluster integrity depends on cholesterol. Here we report that the native lipid environment is not sufficient for correct syntaxin 1 clustering and that additional cytoplasmic protein-protein interactions, primarily involving the SNARE motif, are required. Apparently no specific cofactors are needed because i), clusters form equally well in nonneuronal cells, and ii), as revealed by nanoscale subdiffraction resolution provided by STED microscopy, the number of clusters directly depends on the syntaxin 1 concentration. For syntaxin 4 clustering the N-terminal domain and the linker region are also dispensable. Moreover, clustering is specific because in both cluster types syntaxins mutually exclude one another at endogenous levels. We suggest that the SNARE motifs of syntaxin 1 and 4 mediate specific syntaxin clustering by homooligomerization, thereby spatially separating sites for different biological activities. Thus, syntaxin clustering represents a mechanism of membrane patterning that is based on protein-protein interactions.