The SNARE Motif Contributes to rbet1 Intracellular Targeting and Dynamics Independently of SNARE Interactions

The endoplasmic reticulum/Golgi SNARE rbet1 cycles between the endoplasmic reticulum and Golgi and is essential for cargo transport in the secretory pathway. Although the quaternary SNARE complex containing rbet1 is known to function in membrane fusion, the structural role of rbet1 is unclear. Furthermore, the structural determinants for rbet1 targeting and its cyclical itinerary have not been investigated. We utilized protein interaction assays to demonstrate that the rbet1 SNARE motif plays a structural role similar to the carboxyl-terminal helix of SNAP-25 in the synaptic SNARE complex and demonstrated the importance to SNARE complex assembly of a conserved salt bridge between rbet1 and sec22b. We also examined the potential role of the rbet1 SNARE motif and SNARE interactions in rbet1 localization and dynamics. We found that, in contrast to what has been observed for syntaxin 5, the rbet1 SNARE motif was essential for proper targeting. To test whether SNARE interactions were important for the targeting function of the SNARE motif, we used charge repulsion mutations at the conserved salt bridge position that rendered rbet1 defective for binary, ternary, and quaternary SNARE interactions. We found that heteromeric SNARE interactions are not required at any step in rbet1 targeting or dynamics. Furthermore, the heteromeric state of the SNARE motif does not influence its interaction with the COPI coat or efficient recruitment onto transport vesicles. We conclude that protein targeting is a completely independent function of the rbet1 SNARE motif, which is capable of distinct classes of protein interactions.