Phragmoplastin dynamics: multiple forms,microtubule association and their roles in cell plate formation in plants

We have characterized 4 of the 16 members of the family of dynamin-related proteins (DRP) in Arabidopsis. Three members, DRP1A (previously referred as ADL1), DRP1C and DRP1E, belong to the largest group of phragmoplastin-like proteins. DRP2A (ADL6) is one of the two members that contain a pleckstrin homology (PH) domain and a proline-rich (PR) motif, characteristics of animal dynamins. All four proteins interacted in yeast two-hybrid assays with phragmoplastin, and showed different patterns of localization at the forming cell plate during cytokinesis. GFP-tagged DRP1A and DRP1C proteins were found to be associated with the cytoskeleton in G1 phase of the cell cycle. The distribution pattern of DRP1A was sensitive to propyzamid and insensitive to cytochalasin D, suggesting that DRP1A is associated with microtubules and not actin filaments. The association of DRP1A with microtubules was confirmed in vitro by spin-down assays. A GTPase-defective phragmoplastin acted as a dominant negative mutant, reduced transport of vesicles to the cell plate and formed dense tubule-like structures in the cell plate. We propose that DRP1 proteins may provide an anchor for Golgi-derived vesicles to attach to microtubules, which in turn direct the vesicles to the forming cell plate during cytokinesis. Whereas the DRP1 subfamily members are involved in tubulization of membranes, DRP2 may be involved in endocytosis and membrane recycling via clathrin-coated vesicles.     

Members of the Arabidopsis dynamin-like gene family,ADL1, are essential for plant cytokinesis and polarized cell growth

Polarized membrane trafficking during plant cytokinesis and cell expansion are critical for plant morphogenesis, yet very little is known about the molecular mechanisms that guide this process. Dynamin and dynamin-related proteins are large GTP binding proteins that are involved in membrane trafficking. Here, we show that two functionally redundant members of the Arabidopsis dynamin-related protein family, ADL1A and ADL1E, are essential for polar cell expansion and cell plate biogenesis. adl1A-2 adl1E-1 double mutants show defects in cell plate assembly, cell wall formation, and plasma membrane recycling. Using a functional green fluorescent protein fusion protein, we show that the distribution of ADL1A is dynamic and that the protein is localized asymmetrically to the plasma membrane of newly formed and mature root cells. We propose that ADL1-mediated membrane recycling is essential for plasma membrane formation and maintenance in plants.     

Phragmoplastin, a dynamin-like protein associated with cell plate formation in plants.

Cytokinesis in a plant cell is accomplished by the formation of a cell plate in the center of the phragmoplast. Little is known of the molecular events associated with this process. In this study, we report the identification of a dynamin-like protein from soybean and demonstrate that this protein is associated with the formation of the cell plate. Plant dynamin-like (PDL) protein contains 610 amino acids showing high homology with other members of the dynamin protein family. Western blot experiments demonstrated that it is associated with the non-ionic detergent-resistant fraction of membranes. Indirect immunofluorescence microscopy localized PDL to the cell plate in dividing soybean root tip cells. Double labeling experiments demonstrated that, unlike phragmoplast microtubules which are concentrated on the periphery of the forming plate, PDL is located across the whole width of the newly formed cell plate. Based on the temporal and spatial organization of PDL in the phragmoplast, we termed this protein 'phragmoplastin'. The data suggest that phragmoplastin may be associated with exocytic vesicles that are depositing cell plate material during cytokinesis in the plant cell.     

A novel RNA-binding protein associated with cell plate formation

Building a cell plate during cytokinesis in plant cells requires the participation of a number of proteins in a multistep process. We previously identified phragmoplastin as a cell plate-specific protein involved in creating a tubulovesicular network at the cell plate. We report here the identification and characterization of a phragmoplastin-interacting protein, PHIP1, in Arabidopsis (Arabidopsis thaliana). It contains multiple functional motifs, including a lysine-rich domain, two RNA recognition motifs, and three CCHC-type zinc fingers. Polypeptides with similar motif structures were found only in plant protein databases, but not in the sequenced prokaryotic, fungal, and animal genomes, suggesting that PHIP1 represents a plant-specific RNA-binding protein. In addition to phragmoplastin, two Arabidopsis small GTP-binding proteins, Rop1 and Ran2, are also found to interact with PHIP1. The zinc fingers of PHIP1 were not required for its interaction with Rop1 and phragmoplastin, but they may participate in its binding with the Ran2 mRNA. Immunofluorescence, in situ RNA hybridization, and green fluorescent protein tagging experiments showed the association of PHIP1 with the forming cell plate during cytokinesis. Taken together, our data suggest that PHIP1 is a novel RNA-binding protein and may play a unique role in the polarized mRNA transport to the vicinity of the cell plate.     

The Arabidopsis KNOLLE Protein Is a Cytokinesis-specific Syntaxin

In higher plant cytokinesis, plasma membrane and cell wall originate by vesicle fusion in the plane of cell division. The Arabidopsis KNOLLE gene, which is required for cytokinesis, encodes a protein related to vesicle-docking syntaxins. We have raised specific rabbit antiserum against purified recombinant KNOLLE protein to show biochemically and by immunoelectron microscopy that KNOLLE protein is membrane associated. Using immunofluorescence microscopy, KNOLLE protein was found to be specifically expressed during mitosis and, unlike the plasma membrane H⁺-ATPase, to localize to the plane of division during cytokinesis. Arabidopsis dynamin-like protein ADL1 accumulates at the plane of cell plate formation in knolle mutant cells as in wild-type cells, suggesting that cytokinetic vesicle traffic is not affected. Furthermore, electron microscopic analysis indicates that vesicle fusion is impaired. KNOLLE protein was detected in mitotically dividing cells of various parts of the developing plant, including seedling root, inflorescence meristem, floral meristems and ovules, and the cellularizing endosperm, but not during cytokinesis after the male second meiotic division. Thus, KNOLLE is the first syntaxin-like protein that appears to be involved specifically in cytokinetic vesicle fusion.     

A New Dynamin-Like Protein, ADL6, Is Involved in Trafficking from the trans-Golgi Network to the Central Vacuole in Arabidopsis

Dynamin, a high-molecular-weight GTPase, plays a critical role in vesicle formation at the plasma membrane during endocytosis in animal cells. Here we report the identification of a new dynamin homolog in Arabidopsis named Arabidopsis dynamin-like 6 (ADL6). ADL6 is quite similar to dynamin I in its structural organization: a conserved GTPase domain at the N terminus, a pleckstrin homology domain at the center, and a Pro-rich motif at the C terminus. In the cell, a majority of ADL6 is associated with membranes. Immunohistochemistry and in vivo targeting experiments revealed that ADL6 is localized to the Golgi apparatus. Expression of the dominant negative mutant ADL6[K51E] in Arabidopsis protoplasts inhibited trafficking of cargo proteins destined for the lytic vacuole and caused them to accumulate at the trans-Golgi network. In contrast, expression of ADL6[K51E] did not affect trafficking of a cargo protein, H(+)-ATPase:green fluorescent protein, destined for the plasma membrane. These results suggest that ADL6 is involved in vesicle formation for vacuolar trafficking at the trans-Golgi network but not for trafficking to the plasma membrane in plant cells.     

Dynamics of phragmoplastin in living cells during cell plate formation and uncoupling of cell elongation from the plane of cell division.

The cell plate is formed by the fusion of Golgi apparatus-derived vesicles in the center of the phragmoplast during cytokinesis in plant cells. A dynamin-like protein, phragmoplastin, has been isolated and shown to be associated with cell plate formation in soybean by using immunocytochemistry. In this article, we demonstrate that similar to dynamin, phragmoplastin polymerizes to form oligomers. We fused soybean phragmoplastin with the green fluorescence protein (GFP) and introduced it into tobacco BY-2 cells to monitor the dynamics of early events in cell plate formation. We demonstrate that the chimeric protein is functional and targeted to the cell plate during cytokinesis in transgenic cells. GFP-phragmoplastin was found to appear first in the center of the forming cell plate, and as the cell plate grew outward, it redistributed to the growing margins of the cell plate. The redistribution of phragmoplastin may require microtubule reorganization because the microtubule-stabilizing drug taxol inhibited phragmoplastin redistribution. Our data show that throughout the entire process of cytokinesis, phragmoplastin is concentrated in the area in which membrane fusion is active, suggesting that phragmoplastin participates in an early membrane fusion event during cell plate formation. Based on the dynamics of GFP-phragmoplastin, it appears that the process of cell plate formation is completed in two phases. The first phase is confined to the cylinder of the phragmoplast proper and is followed by a second phase that deposits phragmoplast vesicles in a concentric fashion, resulting in a ring of fluorescence, with the concentration of vesicles being higher at the periphery. In addition, overexpression of GFP-phragmoplastin appears to act as a dominant negative, slowing down the completion of cell plate formation, and often results in an oblique cell plate. The latter appears to uncouple cell elongation from the plane of cell division, forming twisted and elongated cells with longitudinal cell divisions.     

The dynamin-like protein ADL1C is essential for plasma membrane maintenance during pollen maturation

Dynamin-related GTPases regulate a wide variety of dynamic membrane processes in eukaryotes. Here, we investigated the function of ADL1C, a member of the Arabidopsis 68 kDa dynamin-like protein family. Analysis of heterozygous adl1C-1 indicates that the mutation specifically affects post-meiotic male gametogenesis. Fifty percent of the mature pollen from heterozygous adl1C-1 androecia are shriveled and fail to germinate in vitro. During microspore maturation, adl1C-1 pollen grains display defects in the plasma membrane and intine morphology, suggesting that ADL1C is essential for the formation and maintenance of the pollen cell surface and viability during desiccation. Consistent with a role in cell-surface dynamics, immunofluorescence microscopy indicates that ADL1C is localized to the cell plate of dividing somatic cells and to the tip of expanding root hairs. We propose that ADL1C functions in plasma membrane dynamics, and we discuss the role of the ADL1 family in plant growth and development.     

Regulation of ADL6 activity by its associated molecular network

Plant dynamin-like proteins consist of a group of high molecular weight GTPase with diverse structural arrangements and cellular localizations. In addition, unlike animal dynamins, there was no evidence for the involvement of any plant dynamin-like protein in clathrin-mediated vesicle trafficking. In this study we demonstrate that ADL6 (Arabidopsis dynamin-like protein 6), due to its domain arrangement, behaves similarly to the animal dynamins. The association of ADL6 with clathrin-coated vesicles was demonstrated by co-fractionation and immunocytochemical studies. ADL6 also interacted via its C-terminus with gamma-adaptin, an adaptor protein of clathrin-coated vesicles. Our results suggest that ADL6 participates in clathrin-mediated vesicle trafficking originating from the Golgi. In addition, our studies demonstrate that ADL6 intrinsic GTPase activity is regulated by its association with acidic phospholipids and an SH3 (Src homology 3)-containing protein.     

The Arabidopsis dynamin-like proteins ADL1C and ADL1E play a critical role in mitochondrial morphogenesis

Dynamin-related proteins are high molecular weight GTP binding proteins and have been implicated in various biological processes. Here, we report the functional characterization of two dynamin homologs in Arabidopsis, Arabidopsis dynamin-like 1C (ADL1C) and Arabidopsis dynamin-like 1E (ADL1E). ADL1C and ADL1E show a high degree of amino acid sequence similarity with members of the dynamin family. However, both proteins lack the C-terminal Pro-rich domain and the pleckstrin homology domain. Expression of the dominant-negative mutant ADL1C[K48E] in protoplasts obtained from leaf cells caused abnormal mitochondrial elongation. Also, a T-DNA insertion mutation at the ADL1E gene caused abnormal mitochondrial elongation that was rescued by the transient expression of ADL1C and ADL1E in protoplasts. In immunohistochemistry and in vivo targeting experiments in Arabidopsis protoplasts, ADL1C and ADL1E appeared as numerous speckles and the two proteins colocalized. These speckles were partially colocalized with F1-ATPase-gamma:RFP, a mitochondrial marker, and ADL2b localized at the tip of mitochondria. These results suggest that ADL1C and ADL1E may play a critical role in mitochondrial fission in plant cells.     

A novel Arabidopsis thaliana dynamin-like protein containing the pleckstrin homology domain

A full-length cDNA encoding a novel type of plant dynamin-like protein, ADL3, was isolated from Arabidopsis thaliana. ADL3 is a high molecular weight GTPase whose GTP-binding domain shows a low homology to those of other plant dynamin-like proteins. ADL3 contains the pleckstrin homology domain as is in mammalian dynamins, although other plant dynamin-like proteins reported lack this domain. The ADL3 gene was expressed weakly in various tissues, except for siliques with high level expression, which is distinct from the case for other plant dynamin-like protein genes. Taken together, it is predicted that the mode of activation of ADL3 is different from those of other plant homologues.     

KORRIGAN, an Arabidopsis endo-1,4-beta-glucanase, localizes to the cell plate by polarized targeting and is essential for cytokinesis

The formation of the cell plate, a unique structure in dividing plant cells, is pivotal for cytokinesis. A mutation in the Arabidopsis KORRIGAN (KOR) gene causes the formation of aberrant cell plates, incomplete cell walls, and multinucleated cells, leading to severely abnormal seedling morphology. The mutant, designed kor1-2, was identified as a stronger allele than the previously identified kor1-1, which appears to be defective only in cell elongation. KOR1 encodes an endo-1,4-beta-d-glucanase with a transmembrane domain and two putative polarized targeting signals in the cytosolic tail. When expressed in tobacco BY2 cells, a KOR1-GFP (green fluorescence protein) fusion protein was localized to growing cell plates. Substitution mutations in the polarized targeting motifs of KOR1 caused the fusion proteins to localize to the plasma membrane as well. Expression of these mutant genes in kor1-2 plants complemented only the cell elongation defect but not the cytokinesis defect, indicating that polarized targeting of KOR1 to forming cell plates is essential for cytokinesis. Our results suggest that KOR1 plays a critical role during cytokinesis.     

<Emphasis Type="Italic">Arabidopsis</Emphasis> dynamin-related protein DRP2B is co-localized with DRP1A on the leading edge of the forming cell plate

The Arabidopsis genome has six families of dynamin-related proteins. One of these families includes DRP2A and DRP2B. The domain structures of proteins of this family are most similar to those of the animal endocytosis protein, dynamin. In this study, the signals of GFP-tagged DRP2B were strongly detected in the cell plate of Arabidopsis root tip cells and tobacco cultured cells. Time-lapse observations of these signals during cytokinesis in tobacco cultured cells suggested that DRP2B mainly localized to the newly formed part of the cell plate, and that the localization dynamics of DRP2B was quite similar to that of DRP1A, which is an Arabidopsis dynamin-related protein that is closely related to soybean phragmoplastin. These results indicate that Arabidopsis dynamin-related proteins, DRP1A and DRP2B, from two different families, participate in membrane remodeling at a similar place in the cell plate.     

A novel UDP-glucose transferase is part of the callose synthase complex and interacts with phragmoplastin at the forming cell plate

Using phragmoplastin as a bait, we isolated an Arabidopsis cDNA encoding a novel UDP-glucose transferase (UGT1). This interaction was confirmed by an in vitro protein--protein interaction assay using purified UGT1 and radiolabeled phragmoplastin. Protein gel blot results revealed that UGT1 is associated with the membrane fraction and copurified with the product-entrapped callose synthase complex. These data suggest that UGT1 may act as a subunit of callose synthase that uses UDP-glucose to synthesize callose, a 1,3-beta-glucan. UGT1 also interacted with Rop1, a Rho-like protein, and this interaction occurred only in its GTP-bound configuration, suggesting that the plant callose synthase may be regulated by Rop1 through the interaction with UGT1. The green fluorescent protein--UGT1 fusion protein was located on the forming cell plate during cytokinesis. We propose that UGT1 may transfer UDP-glucose from sucrose synthase to the callose synthase and thus help form a substrate channel for the synthesis of callose at the forming cell plate.     

Arabidopsis R-SNARE proteins VAMP721 and VAMP722 are required for cell plate formation

Background

Cell plate formation during plant cytokinesis is facilitated by SNARE complex-mediated vesicle fusion at the cell-division plane. However, our knowledge regarding R-SNARE components of membrane fusion machinery for cell plate formation remains quite limited.

Methodology/Principal Findings

We report the in vivo function of Arabidopsis VAMP721 and VAMP722, two closely sequence-related R-SNAREs, in cell plate formation. Double homozygous vamp721vamp722 mutant seedlings showed lethal dwarf phenotypes and were characterized by rudimentary roots, cotyledons and hypocotyls. Furthermore, cell wall stubs and incomplete cytokinesis were frequently observed in vamp721vamp722 seedlings. Confocal images revealed that green fluorescent protein-tagged VAMP721 and VAMP722 were preferentially localized to the expanding cell plates in dividing cells. Drug treatments and co-localization analyses demonstrated that punctuate organelles labeled with VAMP721 and VAMP722 represented early endosomes overlapped with VHA-a1-labeled TGN, which were distinct from Golgi stacks and prevacuolar compartments. In addition, protein traffic to the plasma membrane, but not to the vacuole, was severely disrupted in vamp721vamp722 seedlings by subcellular localization of marker proteins.

Conclusion/Significance

These observations suggest that VAMP721 and VAMP722 are involved in secretory trafficking to the plasma membrane via TGN/early endosomal compartment, which contributes substantially to cell plate formation during plant cytokinesis.     

Patellin1, a novel Sec14-like protein, localizes to the cell plate and binds phosphoinositides

Membrane trafficking is central to construction of the cell plate during plant cytokinesis. Consequently, a detailed understanding of the process depends on the characterization of molecules that function in the formation, transport, targeting, and fusion of membrane vesicles to the developing plate, as well as those that participate in its consolidation and maturation into a fully functional partition. Here we report the initial biochemical and functional characterization of patellin1 (PATL1), a novel cell-plate-associated protein that is related in sequence to proteins involved in membrane trafficking in other eukaryotes. Analysis of the Arabidopsis genome indicated that PATL1 is one of a small family of Arabidopsis proteins, characterized by a variable N-terminal domain followed by two domains found in other membrane-trafficking proteins (Sec14 and Golgi dynamics domains). Results from immunolocalization and biochemical fractionation studies suggested that PATL1 is recruited from the cytoplasm to the expanding and maturing cell plate. In vesicle-binding assays, PATL1 bound to specific phosphoinositides, important regulators of membrane trafficking, with a preference for phosphatidylinositol(5)P, phosphatidylinositol(4,5)P(2), and phosphatidylinositol(3)P. Taken together, these findings suggest a role for PATL1 in membrane-trafficking events associated with cell-plate expansion or maturation and point to the involvement of phosphoinositides in cell-plate biogenesis.     

Molecular cloning and characterization of patellin1, a novel sec14-related protein, from zucchini (Cucurbita pepo)

A full-length patellin1 (PATL1) cDNA was cloned and characterized from zucchini (Cucurbita pepo). PATL1, originally discovered in the higher plant Arabidopsis thaliana, is a plant Sec14-related protein that localizes to the cell plate during the late stages of cytokinesis. PATL1 is related in sequence to other eukaryotic proteins involved in membrane trafficking and is thought to participate in vesicle trafficking events associated with cell plate maturation. The zucchini PATL1 (CpPATL1) cDNA predicts a 605 amino acid protein which consists of an acidic N-terminal domain (pI=4.2) followed by a Sec14 lipid-binding domain and a C-terminal Golgi dynamics domain (GOLD). The predicted CpPATL1 protein sequence shows a high degree of similarity to Arabidopsis PATL1, especially in the Sec14 (84%) and GOLD domains (87%). A phylogenetic analysis of all available full-length PATL sequences revealed that the PATLs belong to four distinct clades; CpPATL1 is a member of the PATL1/2 clade. RT-PCR analysis showed that the CpPATL1 gene is highly expressed throughout the plant. The domain structure, as well as biochemical fractionation studies, which demonstrated that CpPATL1 is a peripheral membrane protein, support a role in membrane trafficking events.     

Phragmoplastin polymerizes into spiral coiled structures via intermolecular interaction of two self-assembly domains

Phragmoplastin, a high molecular weight GTPase belonging to the dynamin superfamily of proteins, becomes associated with the cell plate during cytokinesis in plants. Growth of the cell plate requires continuous fusion of vesicles, and phragmoplastin appears to play a role in the formation of vesicle-tubule-vesicle structures at the cell plate. In this study, we have demonstrated that two self-assembly domains (SA1 and SA2) are involved in polymerization of phragmoplastin. SA1 is about 42 amino acids long and is located near the N terminus overlapping with the GTP-binding region. SA2, containing at least 24 amino acids, is located in the middle of the molecule outside the GTP-binding domain. Peptides containing either SA1 or SA2 interact efficiently with the full-length phragmoplastin. The SA1 domain of one phragmoplastin molecule also binds to SA2 of another as confirmed in vitro by using radiolabeled peptides. This interaction leads to the formation of polymers with a staggered contoured spiral structure. Electron microscopy studies revealed that helical arrays of phragmoplastin can be induced by reducing salt concentration. Our results suggest that phragmoplastin may assemble into helical arrays that wrap around and squeeze vesicles into vesicle-tubule-vesicle structures observed on the forming cell plate.     

The cytokinesis gene KEULE encodes a Sec1 protein that binds the syntaxin KNOLLE

KEULE is required for cytokinesis in Arabidopsis thaliana. We have positionally cloned the KEULE gene and shown that it encodes a Sec1 protein. KEULE is expressed throughout the plant, yet appears enriched in dividing tissues. Cytokinesis-defective mutant sectors were observed in all somatic tissues upon transformation of wild-type plants with a KEULE-green fluorescent protein gene fusion, suggesting that KEULE is required not only during embryogenesis, but at all stages of the plant's life cycle. KEULE is characteristic of a Sec1 protein in that it appears to exist in two forms: soluble or peripherally associated with membranes. More importantly, KEULE binds the cytokinesis-specific syntaxin KNOLLE. Sec1 proteins are key regulators of vesicle trafficking, capable of integrating a large number of intra- and/or intercellular signals. As a cytokinesis-related Sec1 protein, KEULE appears to represent a novel link between cell cycle progression and the membrane fusion apparatus.