The SNARE motif is essential for the formation of syntaxin clusters in the plasma membrane

Abstract

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.

FIGURE 1

Overexpression of syntaxin 1A in BHK and PC12 cells. (A–C) Syntaxin 1A-GFP clusters in BHK cells lacking endogenous syntaxin 1. A brightly fluorescent cell was selected and disrupted by ultrasound treatment on the microscope stage. Immediately after rupture, an image was taken in the GFP channel (B; for magnified view see C). To rule out the possibility of areas devoid of fluorescence being holes in the plasma membrane, membrane integrity was documented by staining phospholipids with TMA-DPH (A). (D–F) Overexpression of syntaxin 1A-GFP in PC12 cells. Membrane sheets were fixed and immunostained with an antibody visualizing endogenous and overexpressed syntaxin 1 (E; for different scaling see F). As judged from the GFP fluorescence (D), the left membrane sheet contained almost no overexpressed syntaxin 1A-GFP, therefore immunostained clusters arise largely from endogenous syntaxin 1 (E). A highly elevated syntaxin 1 level, as documented for the right membrane sheet, results in a more diffuse appearance (F).

FIGURE 2

STED microscopy reveals a correlation between syntaxin 1 cluster density and expression level. (A and B, upper left) Experimentally determined point spread functions of confocal and STED microscopy. Glass-adsorbed primary antibodies visualized by fluorescently labeled secondary antibodies served as fluorescent point sources. For illustration, signals obtained for several primary antibodies were overlaid (12 for the confocal and 29 for the STED image) and line scans placed through the center of fluorescence. The determined FWHM (see Methods for details) are marked by arrows and approximate the resolution provided by the corresponding imaging technique. For the signal detected by STED microscopy, a FWHM of 72 nm was measured, representing a 7.1-fold reduction in focal area compared to confocal microscopy (FWHM 192 nm). (A and B, upper right) Confocal and STED micrographs from membrane sheets generated from PC12 cells transfected with myc syntaxin 1A. Both endogenous and overexpressed syntaxin 1 were visualized using an antibody recognizing the N-terminal domain of syntaxin 1. The processed images (see Methods for details) show several membrane sheets with varying expression levels ranging from low to high (as indicated in A). (A and B, lower panels) Magnified views from right upper images scaled to visualize individual spots. (C) From STED images as shown in B, we determined the number of fluorescent spots in a defined area and plotted them against the image intensity of the corresponding image (for details see Methods).

FIGURE 3

SNARE motif is mandatory for coclustering. (A) An immunostained membrane sheet generated from a PC12 cell cooverexpressing myc-tagged syntaxin 1A (left, red channel) and syntaxin 1A-GFP (middle, green channel). The right image represents a color overlay of both channels. A fluorescent bead (arrow) acts as a spatial reference, for cross-checking the automated alignment. (B) Magnified views from A (upper panel) and an experiment in which a syntaxin 1A-GFP construct (Sx1A, TMR-GFP) lacking the N-terminal domain, the linker region, and the SNARE motif was used (lower panel) instead of the full-length syntaxin. Arrows indicating some of the bright spots in the red channel were transferred to identical pixel locations in the green channel and the overlay. (C) Correlation analysis of experiments like those illustrated in A and B. To obtain an objective measure for the similarity between the red and the green channel, the Pearson correlation coefficient was calculated (see Methods for details). The images acquired for the myc- or GFP-tagged full-length syntaxin were correlated with the ones obtained for the illustrated syntaxin/syntaxin variants carrying the alternate tag (see Methods for details). Constructs analyzed from left to right: full-length syntaxin 1A (Sx1A-GFP), a construct lacking the N-terminal domain, linker region, and SNARE motif (Sx1A, TMR-GFP), syntaxin 1A lacking the N-terminal domain and linker region (Sx1A, SNARE motif-TMR-GFP), a mutant carrying three mutations abolishing TMR oligomerization (Sx1AmutTMR-GFP), and a construct with mutations preventing the closed conformation of syntaxin 1A (myc Sx1Aopen). For each construct, 4–5 independent experiments were performed. Values are given as mean ± SE.

FIGURE 4

Syntaxin 4 and syntaxin 1 clusters are strictly separated. Double immunostaining for syntaxin 4 (left, red) and syntaxin 1 (middle, green). The lower panel shows magnified views of the corresponding images in the upper panel. Circles were superimposed onto bright fluorescent spots in the syntaxin 4 channel and transferred to identical image locations in the syntaxin 1 channel and the overlay (right). The colocalization of syntaxin 4 and syntaxin 1 was assessed by two independent approaches. Based on morphological criteria, we first determined the fraction of syntaxin 4 clusters colocalizing with syntaxin 1 clusters (for details see Methods) and found no colocalization (0.9% ± 1.5%; n = 3 independent experiments, values are given as mean ± SE). Second, the correlation coefficient of the two channels was calculated to be −0.01 ± 0.01 (n = 3 independent experiments, values are given as mean ± SE; for details see Methods).

FIGURE 5

Coclustering experiment with syntaxin 4. (A) Representative images showing a magnified view of a membrane sheet generated from a PC12 cell overexpressing myc syntaxin 4 and syntaxin 4 GFP. Arrows pointing to bright spots in the red channel (left) were transferred to identical pixel locations in the green channel (middle) and the overlay (right). (B) Correlation analysis. From images like those illustrated in A, the correlation coefficient between the red and the green channel was determined (for details, see Methods). GFP-tagged constructs were tested for correlation with myc syntaxin 4. From left to right: full-length syntaxin 4 (Sx4-GFP), full-length syntaxin 1A (Sx1A-GFP), and a syntaxin 4 construct lacking the N-terminal domain and the linker region (Sx4, SNARE motif-TMR-GFP). For each construct, 4–6 independent experiments were performed. Values are given as mean ± SE.