SNAREs at the traffic junction with signalling, transport and nutrition

SNARE proteins contribute to vesicle targeting and membrane fusion in all eukaryotic cells, and are essential to the mechanics of cell growth and development. Nonetheless, there is clear evidence that SNAREs play additional roles, for example in facilitating cellular signalling and synaptic transmission. I report here that SYP121, an Arabidopsis SNARE found at the plasma membrane, contributes to scaffolds that anchor ion channels within the plane of the membrane, and it is a critical structural element determining the K+ channel gating and K+ mineral nutrition. Work from my laboratory has shown that SYP121 interacts in vitro and in vivo with the regulatory (‘silent’) K+ channel subunit KC1 which assembles with different inward-rectifying K+ channels to affect their activities in the plant.SYP121-KC1 interaction was found to be specific to these two protein partners and to be essential for the activity of K+ channels, channel-mediated K+ uptake at the root epidermis and for growth. Electrophysiological studies have shown changes in channel gating that can only result from an altered conformation of the channel and profound changes in the exposure of the channel voltage sensor to the membrane electric field. Intriguingly, when heterologously expressed on its own AKT1 does yield an inward-rectifying K+ current, but its gating characteristics – and those of AKT1 expressed together with KC1 – differ significantly from the K+ current in vivo unless co-expressed together with SYP121. In short, SYP121 is a missing component of the K+ channel complex and is essential for its gating in the plant. These results demonstrate a wholly unexpected role for a SNARE in eukaryotes as part of protein complex facilitating mineral nutrition and wholly unrelated to signalling or to membrane vesicle traffic