Signal motif-dependent ER export of the Qc-SNARE BET12 interacts with MEMB12 and affects PR1 trafficking in Arabidopsis

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) are well-known for their role in controlling membrane fusion, the final, but crucial step, in vesicular transport in eukaryotes. SNARE proteins contribute to various biological processes including pathogen defense and channel activity regulation, as well as plant growth and development. Precise targeting of SNARE proteins to destined compartments is a prerequisite for their proper functioning. However, the underlying mechanism(s) for SNARE targeting in plants remains obscure. Here, we investigate the targeting mechanism of the Arabidopsis thaliana Qc-SNARE BET12, which is involved in protein trafficking in the early secretory pathway. Two distinct signal motifs that are required for efficient BET12 ER export were identified. Pulldown assays and in vivo imaging implicated that both the COPI and COPII pathways were required for BET12 targeting. Further studies using an ER-export-defective form of BET12 revealed that the Golgi-localized Qb-SNARE MEMB12, a negative regulator of pathogenesis-related protein 1 (PR1; At2g14610) secretion, was its interacting partner. Ectopic expression of BET12 caused no inhibition in the general ER-Golgi anterograde transport but caused intracellular accumulation of PR1, suggesting that BET12 has a regulatory role in PR1 trafficking in A. thaliana.