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

Abstract

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.

Loss of GET pathway orthologs in Arabidopsis thaliana causes root hair growth defects and affects SNARE abundance

“…Of the estimated 500 TA proteins in A. thaliana (13), many are vital for development and survival of the plant. Especially SNARE proteins, which facilitate vesicle fusion to drive processes, such as cytokinesis, pathogen defense, and ion homeostasis (4,7,47), require correct and efficient membrane insertion. Inability of the plant to insert TA proteins should yield severe growth defects at least similar to if not stronger than-for example-the knolle phenotype caused by an syp111 loss of function allele (coding for the Qa-SNARE KNOLLE).…”

“…This vital process guarantees cellular expansion through addition of membrane material, cell plate formation, and cargo delivery (4,5). SNARE proteins are also involved in regulating potassium channels and aquaporins (6)(7)(8).…”

VAMP721 Conformations Unmask an Extended Motif for K+ Channel Binding and Gating Control

“…Regardless of the conformational details, however, it is clear that channel binding by VAMP721 and by its cognate Qa-SNARE SYP121 dovetails within the sequence of events leading to vesicle fusion. Like VAMP721, SYP121 interacts with both the KC1 and KAT1 K + channels, but with opposing effects on channel gating (Honsbein et al, 2009Grefen et al, 2010Grefen et al, , 2015Zhang et al, 2015). This functional juxtaposition between the two cognate SNAREs implies a sequential handover in binding with the K + channels as the two SNAREs assemble a SNARE complex to drive membrane fusion.…”

“…The R-SNAREs VAMP721 and VAMP722 assemble in complex with the plasma membrane Qa-SNARE SYP121 (Karnik et al, 2013b. SYP121 also binds with the K + channels KC1 and KAT1, altering channel gating to promote K + uptake and conferring a voltage dependence to secretory traffic for growth (Honsbein et al, 2009Grefen et al, 2010Grefen et al, , 2015. Binding with SYP121 has been shown to promote vesicle traffic with osmotic solute uptake, including that of K + , effectively coordinating the two processes and maintaining turgor as the cell expands (Karnik et al, 2017).…”

Techniques for the analysis of protein-protein interactions in vivo

“…The split-ubiquitin system has been used extensively in plant research to identify interactions of membrane proteins such as G-proteins (Aranda-Sicilia et al, 2015), phosphate transporters (Fontenot et al, 2015), and aquaporins (Besserer et al, 2012;Hachez et al, 2014), potassium channels (Obrdlik et al, 2004), and their interactions with SNARE proteins (Honsbein et al, 2009;Grefen et al, 2015;Zhang et al, 2015), specifically using the vectors modified by Petr Obrdlik and coworkers (Obrdlik et al, 2004). Improvements to the original vector system included increasing the signal-to-noise ratio through introducing a Met-repressible promoter driving the bait Cub fusion (Obrdlik et al, 2004), incorporating cloning sites for in vivo cloning as well as Gateway recombination technology (Obrdlik et al, 2004;Grefen et al, 2007Grefen et al, , 2009, and altering linkers and tags (Grefen and Blatt, 2012b), all of which, together with the development of strains that allow a mating-based approach to be taken (Ludewig et al, 2003), have made the system suitable for high-throughput analysis (Box I; Lalonde et al, 2010;Chen et al, 2012b;Jones et al, 2014).…”