Dual Sites for SEC11 on the SNARE SYP121 Implicate a Binding Exchange during Secretory Traffic

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins facilitate vesicle traffic through their assembly in a heteromeric complex that drives membrane fusion. Much of vesicle traffic at the Arabidopsis (Arabidopsis thaliana) plasma membrane is subject to the Sec1/Munc18 protein SEC11, which, along with plasma membrane K(+) channels, selectively binds with the SNARE SYP121 to regulate its assembly in complex. How SEC11 binding is coordinated with the K(+) channels is poorly understood, as both SEC11 and the channels are thought to compete for the same SNARE binding site. Here, we identify a second binding motif within the N terminus of SYP121 and demonstrate that this motif affects SEC11 binding independently of the F(9)xRF motif that is shared with the K(+) channels. This second, previously unrecognized motif is centered on residues R(20)R(21) of SYP121 and is essential for SEC11 interaction with SYP121. Mutation of the R(20)R(21) motif blocked vesicle traffic without uncoupling the effects of SYP121 on solute and K(+) uptake associated with the F(9)xRF motif; the mutation also mimicked the effects on traffic block observed on coexpression of the dominant-negative SEC11(Δ149) fragment. We conclude that the R(20)R(21) motif represents a secondary site of interaction for the Sec1/Munc18 protein during the transition of SYP121 from the occluded to the open conformation that leads to SNARE complex assembly.