A tripartite SNARE-K+ channel complex involved in Arabidopsis potassium nutrition

In addition to their roles in vesicle delivery and fusion, a few membrane trafficking (SNARE) proteins interact with ion channels, notably mammalian Syntaxin1A which binds to specific Ca2+ and K+ channels in nerves and neuroendocrine tissues to modulate their gating properties. Such interactions have been thought to be restricted to mammalian tissues in which they serve highly specialized roles to facilitate signaling and its coupling to membrane traffic. We discovered that the SNARE protein SYP121 of the model plant Arabidopsis binds directly and selectively to the regulatory (‘silent') K+ channel subunit KC1, which assembles with different inward-rectifying Shaker K+ channels to affect their activities. The Shaker subunits AKT1 and KC1 form heterotetramers that are involved in potassium uptake at the root hair and epidermis. We found that SYP121 promotes gating of the inward-rectifying K+ channel AKT1 when heterologously co-expressed with KC1, and that the SYP121–KC1 complex is essential in vivo for AKT1-associated K+ current, channel-mediated K+ uptake at the root epidermis and for growth. These results demonstrate a role for a SNARE as part of protein complex facilitating plant mineral nutrition and they implicate additional roles for SNARE binding to control the activity of other ion channels through the common KC1 subunit