K⁺ Channel-SEC11 Binding Exchange Regulates SNARE Assembly for Secretory Traffic

Abstract

Cell expansion requires that ion transport and secretory membrane traffic operate in concert. Evidence from Arabidopsis (Arabidopsis thaliana) indicates that such coordination is mediated by physical interactions between subsets of so-called SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, which drive the final stages of vesicle fusion, and K 1 channels, which facilitate uptake of the cation to maintain cell turgor pressure as the cell expands. However, the sequence of SNARE binding with the K 1 channels and its interweaving within the events of SNARE complex assembly for exocytosis remains unclear. We have combined protein-protein interaction and electrophysiological analyses to resolve the binding interactions of the hetero-oligomeric associations. We find that the RYxxWE motif, located within the voltage sensor of the K 1 channels, is a nexus for multiple SNARE interactions. Of these, K 1 channel binding and its displacement of the regulatory protein SEC11 is critical to prime the Qa-SNARE SYP121. Our results indicate a stabilizing role for the Qbc-SNARE SNAP33 in the Qa-SNARE transition to SNARE complex assembly with the R-SNARE VAMP721. They also suggest that, on its own, the R-SNARE enters an anomalous binding mode with the channels, possibly as a fail-safe measure to ensure a correct binding sequence. Thus, we suggest that SYP121 binding to the K 1 channels serves the role of a primary trigger to initiate assembly of the secretory machinery for exocytosis.

Synergy among Exocyst and SNARE Interactions Identifies a Functional Hierarchy in Secretion during Vegetative Growth

“…The exocyst forms a peripheral membrane complex (Fendrych et al, 2013;Synek et al, 2014;Kulich et al, 2018). The Qa-and R-SNAREs are integral membrane proteins; whereas SYP121 localizes to the plasma membrane (Lipka et al, 2007;Honsbein et al, 2009;Grefen et al, 2015;Waghmare et al, 2019), the R-SNAREs, including VAMP721, localize to the vesicle membrane and associate transiently with the plasma membrane (Uemura et al, 2005;Zhang et al, 2011Zhang et al, , 2015. We therefore asked whether the loss of one interacting partner could affect the localization of the other as a possible mechanism to explain secretory block.…”

Complex roles of Rabs and SNAREs in the secretory pathway and plant development: a never‐ending story

“…In addition, Zhang and colleagues discovered that the R-SNARE AtVAMP721 also interacts with KAT1 but also with another K + -channel, KC1, indicating a tight regulation between the K + -channels and the AtSYP121/AtVAMP721 SNARE complex (Zhang et al, 2015(Zhang et al, , 2017. Finally, Waghmare et al (2019) determined that the Qb,c-SNARE AtSNAP33 stabilises the AtSYP121/AtVAMP721 SNARE complex after priming of the Qa-SNARE AtSYP121 through its interaction with K + -channel. It was suggested that this binding could correspond to a primary state for initiating the secretory/fusion machinery for exocytosis.…”

“…Finally, Waghmare et al . (2019) determined that the Qb,c‐SNARE AtSNAP33 stabilises the AtSYP121/AtVAMP721 SNARE complex after priming of the Qa‐SNARE AtSYP121 through its interaction with K + ‐channel. It was suggested that this binding could correspond to a primary state for initiating the secretory/fusion machinery for exocytosis.…”

The bare necessities of plant K+ channel regulation

“…We now know that, along with SYP121, the Qbc-SNARE SNAP33 and the R-SNARE VAMP721 bind to the channel VSD domain through a conserved RYxxWE motif located at the N-terminal cytosolic end of the channel and close to the S1 α-helix ( Figure 5 ; Grefen et al, 2015 ; Zhang et al, 2015 ; Waghmare et al, 2019 ). Additionally, the channel binds with the regulatory protein SEC11, a member of the SEC1/MUNC18 protein family that regulates SNARE assembly and secretory traffic ( Karnik et al, 2013 , 2015 ; Waghmare et al, 2019 ).…”