Functional diversification of Arabidopsis SEC1-related SM proteins in cytokinetic and secretory membrane fusion.

Sec1/Munc18 (SM) proteins contribute to membrane fusion by interacting with Qa-SNAREs or nascent trans-SNARE complexes. Gymnosperms and the basal angiosperm Amborella have only a single SEC1 gene related to the KEULE gene in Arabidopsis However, the genomes of most angiosperms including Arabidopsis encode three SEC1-related SM proteins of which only KEULE has been functionally characterized as interacting with the cytokinesis-specific Qa-SNARE KNOLLE during cell-plate formation. Here we analyze the closest paralog of KEULE named SEC1B. In contrast to the cytokinesis defects of keule mutants, sec1b mutants are homozygous viable. However, the keule sec1b double mutant was nearly gametophytically lethal, displaying collapsed pollen grains, which suggests substantial overlap between SEC1B and KEULE functions in secretion-dependent growth. SEC1B had a strong preference for interaction with the evolutionarily ancient Qa-SNARE SYP132 involved in secretion and cytokinesis, whereas KEULE interacted with both KNOLLE and SYP132. This differential interaction with Qa-SNAREs is likely conferred by domains 1 and 2a of the two SM proteins. Comparative analysis of all four possible combinations of the relevant SEC1 Qa-SNARE double mutants revealed that in cytokinesis, the interaction of SEC1B with KNOLLE plays no role, whereas the interaction of KEULE with KNOLLE is prevalent and functionally as important as the interactions of both SEC1B and KEU with SYP132 together. Our results suggest that functional diversification of the two SEC1-related SM proteins during angiosperm evolution resulted in enhanced interaction of SEC1B with Qa-SNARE SYP132, and thus a predominant role of SEC1B in secretion.

Arabidopsis µ-adaptin subunit AP1M of adaptor protein complex 1 mediates late secretory and vacuolar traffic and is required for growth.

Adaptor protein (AP) complexes are the predominant coat proteins of membrane vesicles in post-Golgi trafficking of mammalian cells. Each AP complex contains a specific medium subunit, µ-adaptin, that selects cargo proteins bearing sequence-specific sorting motifs. Much less is known about the AP complexes and their µ subunits in plants. Because of uncertain homology, the µ-adaptins of Arabidopsis have been designated muA through muD [Happel et al. (2004) Plant J 37(5):678-693]. Furthermore, only muD has been assigned to a specific AP complex, AP-3, involved in Golgi-vacuolar trafficking [Niihama et al. (2009) Plant Cell Physiol 50(12):2057-2068, Zwiewka et al. (2011) Cell Res 21(12):1711-1722, and Wolfenstetter et al. (2012) Plant Cell 24(1):215-232]. In contrast, the µ subunit of neither the post-Golgi trafficking AP-1 complex nor the endocytic AP-2 complex has been identified. Here, we report the functional analysis of redundant AP-1 µ-adaptins AP1M1 (also known as muB1) and AP1M2 (also known as muB2). Coimmunoprecipitation revealed that both AP1M2 and its less strongly expressed isoform AP1M1 are complexed with the large subunit γ-adaptin of AP-1. In addition, AP1M2 was localized at or near the trans-Golgi network. Knockout mutations of AP1M2 impaired pollen function and arrested plant growth whereas the ap1m1 ap1m2 double mutant was nearly pollen-lethal. At the cellular level, the absence of AP1M2 entailed inhibition of multiple trafficking pathways from the trans-Golgi network to the vacuole and to the plasma membrane in interphase and to the plane of cell division in cytokinesis. Thus, AP-1 is crucial in post-Golgi trafficking in plant cells and required for cell division and plant growth.

Plant cytokinesis: a tale of membrane traffic and fusion.

Cytokinesis separates the forming daughter cells. Higher plants have lost the ability to constrict the plasma membrane (PM) in the division plane. Instead, trans-Golgi network (TGN)-derived membrane vesicles are targeted to the centre of the division plane and generate, by homotypic fusion, the partitioning membrane named cell plate (CP). The CP expands in a centrifugal fashion until its margin fuses with the PM at the cortical division site. Mutant screens in Arabidopsis have identified a cytokinesis-specific syntaxin named KNOLLE and an interacting Sec1/Munc18 (SM) protein named KEULE both of which are required for vesicle fusion during cytokinesis. KNOLLE is only made during M-phase, targeted to the division plane and degraded in the vacuole at the end of cytokinesis. Here we address mechanisms of KNOLLE trafficking and interaction of KNOLLE with different soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) partners and with SM-protein KEULE, ensuring membrane fusion in cytokinesis.

NSF/αSNAP2-mediated cis-SNARE complex disassembly precedes vesicle fusion in Arabidopsis cytokinesis.

Eukaryotic membrane fusion requires trans-SNARE complexes bridging the gap between adjacent membranes1. Fusion between a transport vesicle and its target membrane transforms the trans- into a cis-SNARE complex. The latter interacts with the hexameric AAA+-ATPase N-ethylmaleimide-sensitive factor (NSF) and its co-factor alpha-soluble NSF attachment protein (αSNAP), forming a 20S complex2,3. ATPase activity disassembles the SNAP receptor (SNARE) complex into Qa-SNARE, which folds back onto itself, and its partners4,5. The fusion of identical membranes has a different sequence of events6. The fusion partners each have cis-SNARE complexes to be broken up by NSF and αSNAP. The Qa-SNARE monomers are then stabilized by interaction with Sec1/Munc18-type regulators (SM proteins) to form trans-SNARE complexes, as shown for the yeast vacuole7. Membrane fusion in Arabidopsis cytokinesis is formally akin to vacuolar fusion8. Membrane vesicles fuse with one another to form the partitioning membrane known as the cell plate. Cis-SNARE complexes of cytokinesis-specific Qa-SNARE KNOLLE and its SNARE partners are assembled at the endoplasmic reticulum and delivered by traffic via the Golgi/trans-Golgi network to the cell division plane9. The SM protein KEULE is required for the formation of trans-SNARE complexes between adjacent membrane vesicles10. Here we identify NSF and its adaptor αSNAP2 as necessary for the disassembly of KNOLLE cis-SNARE complexes, which is a prerequisite for KNOLLE-KEULE interaction in cytokinesis. In addition, we show that NSF is required for other trafficking pathways and interacts with the respective Q-SNAREs. The SNARE complex disassembly machinery is conserved in plants and plays a unique essential role in cytokinesis.

AtRMR1 functions as a cargo receptor for protein trafficking to the protein storage vacuole.

Organellar proteins are sorted by cargo receptors on the way to their final destination. However, receptors for proteins that are destined for the protein storage vacuole (PSV) are largely unknown. In this study, we investigated the biological role that Arabidopsis thaliana receptor homology region transmembrane domain ring H2 motif protein (AtRMR) 1 plays in protein trafficking to the PSV. AtRMR1 mainly colocalized to the prevacuolar compartment of the PSV, but a minor portion also localized to the Golgi complex. The coexpression of AtRMR1 mutants that were localized to the Golgi complex strongly inhibited the trafficking of phaseolin to the PSV and caused accumulation of phaseolin in the Golgi complex or its secretion. Co-immunoprecipitation and in vitro binding assays revealed that the lumenal domain of AtRMR1 interacts with the COOH-terminal sorting signal of phaseolin at acidic pH. Furthermore, phaseolin colocalized with AtRMR1 on its way to the PSV. Based on these results, we propose that AtRMR1 functions as the sorting receptor of phaseolin for its trafficking to the PSV.

Concerted Action of Evolutionarily Ancient and Novel SNARE Complexes in Flowering-Plant Cytokinesis.

Membrane vesicles delivered to the cell-division plane fuse with one another to form the partitioning membrane during plant cytokinesis, starting in the cell center. In Arabidopsis, this requires SNARE complexes involving the cytokinesis-specific Qa-SNARE KNOLLE. However, cytokinesis still occurs in knolle mutant embryos, suggesting contributions from KNOLLE-independent SNARE complexes. Here we show that Qa-SNARE SYP132, having counterparts in lower plants, functionally overlaps with the flowering plant-specific KNOLLE. SYP132 mutation causes cytokinesis defects, knolle syp132 double mutants consist of only one or a few multi-nucleate cells, and SYP132 has the same SNARE partners as KNOLLE. SYP132 and KNOLLE also have non-overlapping functions in secretion and in cellularization of the embryo-nourishing endosperm resulting from double fertilization unique to flowering plants. Evolutionarily ancient non-specialized SNARE complexes originating in algae were thus amended by the appearance of cytokinesis-specific SNARE complexes, meeting the high demand for membrane-fusion capacity during endosperm cellularization in angiosperms.

ER assembly of SNARE complexes mediating formation of partitioning membrane in Arabidopsis cytokinesis.

Intracellular membrane fusion mediates diverse processes including cell growth, division and communication. Fusion involves complex formation between SNARE proteins anchored to adjacent membranes. How and in what form interacting SNARE proteins reach their sites of action is virtually unknown. We have addressed this problem in the context of plant cell division in which a large number of TGN-derived membrane vesicles fuse with one another to form the partitioning membrane. Blocking vesicle formation at the TGN revealed cis-SNARE complexes. These inactive cytokinetic SNARE complexes were already assembled at the endoplasmic reticulum and, after passage through Golgi/TGN to the cell division plane, transformed into fusogenic SNARE complexes. This mode of trafficking might ensure delivery of large stoichiometric quantities of SNARE proteins required for forming the partitioning membrane in the narrow time frame of plant cytokinesis. Such long-distance trafficking of inactive SNARE complexes would also facilitate directional growth processes during cell differentiation.

Delivery of endocytosed proteins to the cell-division plane requires change of pathway from recycling to secretion.

Membrane trafficking is essential to fundamental processes in eukaryotic life, including cell growth and division. In plant cytokinesis, post-Golgi trafficking mediates a massive flow of vesicles that form the partitioning membrane but its regulation remains poorly understood. Here, we identify functionally redundant Arabidopsis ARF guanine-nucleotide exchange factors (ARF-GEFs) BIG1-BIG4 as regulators of post-Golgi trafficking, mediating late secretion from the trans-Golgi network but not recycling of endocytosed proteins to the plasma membrane, although the TGN also functions as an early endosome in plants. In contrast, BIG1-4 are absolutely required for trafficking of both endocytosed and newly synthesized proteins to the cell-division plane during cytokinesis, counteracting recycling to the plasma membrane. This change from recycling to secretory trafficking pathway mediated by ARF-GEFs confers specificity of cargo delivery to the division plane and might thus ensure that the partitioning membrane is completed on time in the absence of a cytokinesis-interphase checkpoint. DOI: http://dx.doi.org/10.7554/eLife.02131.001.

Sec1/Munc18 protein stabilizes fusion-competent syntaxin for membrane fusion in Arabidopsis cytokinesis.

Intracellular membrane fusion requires complexes of syntaxins with other SNARE proteins and regulatory Sec1/Munc18 (SM) proteins. In membrane fusion mediating, e.g., neurotransmitter release or glucose-stimulated insulin secretion in mammals, SM proteins preferentially interact with the inactive closed, rather than the active open, conformation of syntaxin or with the assembled SNARE complex. Other membrane fusion processes such as vacuolar fusion in yeast involve like membranes carrying cis-SNARE complexes, and the role of SM protein is unknown. We investigated syntaxin-SM protein interaction in membrane fusion of Arabidopsis cytokinesis, which involves cytokinesis-specific syntaxin KNOLLE and SM protein KEULE. KEULE interacted with an open conformation of KNOLLE that complemented both knolle and keule mutants. This interaction occurred at the cell division plane and required the KNOLLE linker sequence between helix Hc and SNARE domain. Our results suggest that in cytokinesis, SM protein stabilizes the fusion-competent open form of syntaxin, thereby promoting trans-SNARE complex formation.

Membrane traffic and fusion at post-Golgi compartments.

Complete sequencing of the Arabidopsis genome a decade ago has facilitated the functional analysis of various biological processes including membrane traffic by which many proteins are delivered to their sites of action and turnover. In particular, membrane traffic between post-Golgi compartments plays an important role in cell signaling, taking care of receptor-ligand interaction and inactivation, which requires secretion, endocytosis, and recycling or targeting to the vacuole for degradation. Here, we discuss recent studies that address the identity of post-Golgi compartments, the machinery involved in traffic and fusion or functionally characterized cargo proteins that are delivered to or pass through post-Golgi compartments. We also provide an outlook on future challenges in this area of research.

Actin filaments play a critical role in vacuolar trafficking at the Golgi complex in plant cells.

Actin filaments are thought to play an important role in intracellular trafficking in various eukaryotic cells. However, their involvement in intracellular trafficking in plant cells has not been clearly demonstrated. Here, we investigated the roles actin filaments play in intracellular trafficking in plant cells using latrunculin B (Lat B), an inhibitor of actin filament assembly, or actin mutants that disrupt actin filaments when overexpressed. Lat B and actin2 mutant overexpression inhibited the trafficking of two vacuolar reporter proteins, sporamin:green fluorescent protein (GFP) and Arabidopsis thaliana aleurain-like protein:GFP, to the central vacuole; instead, a punctate staining pattern was observed. Colocalization experiments with various marker proteins indicated that these punctate stains corresponded to the Golgi complex. The A. thaliana vacuolar sorting receptor VSR-At, which mainly localizes to the prevacuolar compartment, also accumulated at the Golgi complex in the presence of Lat B. However, Lat B had no effect on the endoplasmic reticulum (ER) to Golgi trafficking of sialyltransferase or retrograde Golgi to ER trafficking. Lat B also failed to influence the Golgi to plasma membrane trafficking of H+-ATPase:GFP or the secretion of invertase:GFP. Based on these observations, we propose that actin filaments play a critical role in the trafficking of proteins from the Golgi complex to the central vacuole.

Identification of the protein storage vacuole and protein targeting to the vacuole in leaf cells of three plant species.

Protein storage vacuoles (PSVs) are specialized vacuoles devoted to the accumulation of large amounts of protein in the storage tissues of plants. In this study, we investigated the presence of the storage vacuole and protein trafficking to the compartment in cells of tobacco (Nicotiana tabacum), common bean (Phaseolus vulgaris), and Arabidopsis leaf tissue. When we expressed phaseolin, the major storage protein of common bean, or an epitope-tagged version of alpha-tonoplast intrinsic protein (alpha-TIP, a tonoplast aquaporin of PSV), in protoplasts derived from leaf tissues, these proteins were targeted to a compartment ranging in size from 2 to 5 microm in all three plant species. Most Arabidopsis leaf cells have one of these organelles. In contrast, from one to five these organelles occurred in bean and tobacco leaf cells. Also, endogenous alpha-TIP is localized in a similar compartment in untransformed leaf cells of common bean and is colocalized with transiently expressed epitope-tagged alpha-TIP. In Arabidopsis, phaseolin contained N-glycans modified by Golgi enzymes and its traffic was sensitive to brefeldin A. However, trafficking of alpha-TIP was insensitive to brefeldin A treatment and was not affected by the dominant-negative mutant of AtRab1. In addition, a modified alpha-TIP with an insertion of an N-glycosylation site has the endoplasmic reticulum-type glycans. Finally, the early step of phaseolin traffic, from the endoplasmic reticulum to the Golgi complex, required the activity of the small GTPase Sar1p, a key component of coat protein complex II-coated vesicles, independent of the presence of the vacuolar sorting signal in phaseolin. Based on these results, we propose that the proteins we analyzed are targeted to the PSV or equivalent organelle in leaf cells and that proteins can be transported to the PSV by two different pathways, the Golgi-dependent and Golgi-independent pathways, depending on the individual cargo proteins.