Sequential restriction of SNARE-mediated fusion by the COPII inner coat and SNARE chaperones
Mackey, E. J.; Takenaka, T.; Miller, E. A.; Merz, A. J.
Show abstract
SNARE-mediated fusion requires assembly of four SNARE domains (R, Qa, Qb, and Qc) distributed across two membranes, with an SM (Sec1/Munc-18 family) chaperone catalyzing this assembly. We investigated topological requirements for the four SNAREs that mediate ER-Golgi fusion using in vitro assays. In the presence of the cognate SM Sly1, we find that only a single topology drives efficient fusion: the R SNARE on one membrane versus Qa, Qb, and Qc SNAREs on the other. These results prompted us to look upstream, at COPII-SNARE interactions. The assembled COPII coat is known to block fusion. We discovered that of five COPII core subunits, the Sar1 GTPase and Sec23/Sec24 subunits were necessary and sufficient to prevent fusion. When specific Sec24-SNARE interactions were disrupted, fusion was restored, suggesting that SNARE sequestration by COPII prevents fusion. These observations help to explain how appropriate fusion events are facilitated while inappropriate events are deterred.
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