A vesicle-trafficking protein commandeers Kv channel voltage sensors for voltage-dependent secretion

Growth in plants depends on ion transport for osmotic solute uptake and secretory membrane trafficking to deliver material for wall remodelling and cell expansion. The coordination of these processes lies at the heart of the question, unresolved for more than a century, of how plants regulate cell volume and turgor. Here we report that the SNARE protein SYP121 (SYR1/PEN1), which mediates vesicle fusion at the Arabidopsis plasma membrane, binds the voltage sensor domains (VSDs) of K+ channels to confer a voltage dependence on secretory traffic in parallel with K+ uptake. VSD binding enhances secretion in vivo subject to voltage, and mutations affecting VSD conformation alter binding and secretion in parallel with channel gating, net K+ concentration, osmotic content and growth. These results demonstrate a new and unexpected mechanism for secretory control, in which a subset of plant SNAREs commandeer K+ channel VSDs to coordinate membrane trafficking with K+ uptake for growth.