Mechanisms of Functional Specificity Among Plasma‐Membrane Syntaxins in Arabidopsis

Abstract

Syntaxins and interacting SNARE proteins enable membrane fusion in diverse trafficking pathways. The Arabidopsis SYP1 family of plasma membrane-localized syntaxins comprises nine members, of which KNOLLE and PEN1 play specific roles in cytokinesis and innate immunity, respectively. To identify mechanisms conferring specificity of action, we examined one member of each subfamily -KNOLLE/SYP111, PEN1/SYP121 and SYP132 -in regard to subcellular localization, dynamic behavior and complementation of knolle and pen1 mutants when expressed from the same promoters. Our results suggest that cytokinesis-specific syntaxin requires high-level accumulation during cell-plate formation, which necessitates de novo synthesis rather than endocytosis of pre-made protein from the plasma membrane. In contrast, syntaxin in innate immunity does not need upregulation of expression but instead requires pathogen-induced and endocytosis-dependent retargeting to the infection site. This feature of PEN1 is not afforded by SYP132. Additionally, PEN1 could not substitute for KNOLLE because of SNARE domain differences, as revealed by protein chimeras. In contrast, SYP132 was able to rescue knolle as did KNOLLE-SYP132 chimeras. Unlike KNOLLE and PEN1, which appear to have evolved to perform specialized functions, SYP132 stably localized at the plasma membrane and thus might play a role in constitutive membrane fusion. SNARE proteins constitute a family of membraneanchored proteins that play key roles in membrane fusion events of intracellular trafficking pathways by forming SNARE complexes that dock membranes to be fused. Their main characteristic feature is an evolutionarily conserved domain of 60-70 amino acids arranged in heptad repeats, which has been designated the SNARE domain (1). Based on the conserved amino-acid residue at the center of the SNARE domain, SNARE proteins have been classified into R-(arginine) and Q-(glutamine) SNAREs. The Q-SNARE family is further divided into four subfamilies (Qa-, Qb-, Qc-and Qb,cSNAREs) based on differences in the structure of the SNARE domain (2). Each SNARE complex is formed by association of four interacting SNARE domains, one each from VAMP/R-SNARE on the donor membrane and three from Q-SNAREs on the acceptor membrane: one from syntaxin/Qa-SNARE and two from either SNAP25/Qb,c-SNARE or one each from two t-SNARE light chains/Qband Qc-SNAREs (1).

VPS9a Activates the Rab5 GTPase ARA7 to Confer Distinct Pre- and Postinvasive Plant Innate Immunity

“…To our knowledge, ARA6 QL and ARA7 QL are the first reported markers that can distinguish these defense structures, underlining that they differ in composition, although similar in appearance. In line with the result that ARA7 QL did not affect penetration resistance in the wild type ( Figure 1B), this GTP-locked GTPase neither affected RFP-PEN1 accumulation in papillae nor in encasements when this syntaxin was expressed from the S-phase specific histone H4 promoter (Reichardt et al, 2011;Figures 5A and 5B). As these studies are made in mature epidermal cells, in which only previously expressed RFP-PEN1 is found, this result shows that recycled membrane material is delivered to both structures.…”

Myosins XI modulate host cellular responses and penetration resistance to fungal pathogens

“…However, it is important to note that Arabidopsis PEN1 is associated with the pathogen-induced PM microdomains and exhibits constitutive cycling between the PM and endosomes (6,9,43). This striking dynamic behavior may facilitate rapid retargeting of PEN1 to the pathogen infection site.…”

Selective Regulation of Maize Plasma Membrane Aquaporin Trafficking and Activity by the SNARE SYP121

“…Brefeldin A blocks the plasma membrane trafficking of At-PIP2;1 in Arabidopsis roots (Paciorek et al, 2005) and phenocopies the KAT1 K + channel trafficking phenotype in the syp121 mutant (Eisenach et al, 2012). Indeed, SYP121 cycles actively between the TGN and the plasma membrane (Reichardt et al, 2011). In addition, the mCFP:ZmSYP121-Sp2 signal continuously rose during our experiments, demonstrating that the global protein synthesis is not inhibited and did not constitute the origin of the decrease in mYFP: ZmPIP2;5 plasma membrane fluorescence (see Supplemental Figure 4 online).…”