Characterization of Grp1p, a novel cis-Golgi matrix protein

The Golgin Protein RUD3 Regulates Fusarium graminearum Growth and Virulence

Golgins are coiled-coil proteins that play prominent roles in maintaining the structure and function of the Golgi complex. However, the role of golgin proteins in phytopathogenic fungi remains poorly understood. In this study, we functionally characterized the Fusarium graminearum golgin protein RUD3, a homolog of _ScRUD3/GMAP-210 in Saccharomyces cerevisiae and mammalian cells. Cellular localization observation revealed that RUD3 is located in the cis-Golgi. Deletion of RUD3 caused defects in vegetative growth, ascospore discharge, deoxynivalenol (DON) production, and virulence. Moreover, the Δrud3 mutant showed reduced expression of tri genes and impairment of the formation of toxisomes, both of which play essential roles in DON biosynthesis. We further used green fluorescent protein (GFP)-tagged SNARE protein SEC22 (SEC22-GFP) as a tool to study the transport between the endoplasmic reticulum (ER) and Golgi and observed that SEC22-GFP was retained in the cis-Golgi in the Δrud3 mutant. RUD3 contains the coiled coil (CC), GRAB-associated 2 (GA2), GRIP-related Arf binding (GRAB), and GRAB-associated 1 (GA1) domains, which except for GA1, are indispensable for normal localization and function of RUD3, whereas only CC is essential for normal RUD3-RUD3 interaction. Together, these results demonstrate how the golgin protein RUD3 mediates retrograde trafficking in the ER-to-Golgi pathway and is necessary for growth, ascospore discharge, DON biosynthesis, and pathogenicity in F. graminearum IMPORTANCE Fusarium head blight (FHB) caused by the fungal pathogen Fusarium graminearum is an economically important disease of wheat and other small grain cereal crops worldwide, and limited effective control strategies are available. A better understanding of the regulation mechanisms of F. graminearum development, deoxynivalenol (DON) biosynthesis, and pathogenicity is therefore important for the development of effective control management of this disease. Golgins are attached via their extreme carboxy terminus to the Golgi membrane and are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. In this study, we systematically characterized a highly conserved Golgin protein, RUD3, and found that it is required for vegetative growth, ascospore discharge, DON production, and pathogenicity in F. graminearum Our findings provide a comprehensive characterization of the golgin family protein RUD3 in plant-pathogenic fungus, which could help to identify a new potential target for effective control of this devastating disease.

Conserved juxtamembrane domains in the yeast golgin Coy1 drive assembly of a megadalton-sized complex and mediate binding to tethering and SNARE proteins

The architecture and organization of the Golgi complex depend on a family of coiled-coil proteins called golgins. Golgins are thought to form extended homodimers that are C-terminally anchored to Golgi membranes, whereas their N termini extend into the cytoplasm to initiate vesicle capture. Previously, we reported that the Saccharomyces cerevisiae golgin Coy1 contributes to intra-Golgi retrograde transport and binds to the conserved oligomeric Golgi (COG) complex and multiple retrograde Golgi Q-SNAREs (where SNARE is soluble NSF-attachment protein receptor). Here, using various engineered yeast strains, membrane protein extraction and fractionation methods, and in vitro binding assays, we mapped the Coy1 regions responsible for these activities. We also report that Coy1 assembles into a megadalton-size complex and that assembly of this complex depends on the most C-terminal coiled-coil and a conserved region between this coiled-coil and the transmembrane domain of Coy1. We found that this conserved region is necessary and sufficient for binding the SNARE protein Sed5 and the COG complex. Mutagenesis of conserved arginine residues within the C-terminal coiled-coil disrupted oligomerization, binding, and function of Coy1. Our findings indicate that the stable incorporation of Coy1 into a higher-order oligomer is required for its interactions and role in maintaining Golgi homeostasis. We propose that Coy1 assembles into a docking platform that directs COG-bound vesicles toward cognate SNAREs on the Golgi membrane.

The Golgin protein Coy1 functions in intra-Golgi retrograde transport and interacts with the COG complex and Golgi SNAREs

Extended coiled-coil proteins of the Golgin family play prominent roles in maintaining the structure and function of the Golgi complex. Here we further investigate the Golgin protein Coy1 and document its function in retrograde transport between early Golgi compartments. Cells that lack Coy1 displayed a reduced half-life of the Och1 mannosyltransferase, an established cargo of intra-Golgi retrograde transport. Combining the coy1Δ mutation with deletions in other putative retrograde Golgins (sgm1Δ and rud3Δ) caused strong glycosylation and growth defects and reduced membrane association of the Conserved Oligomeric Golgi complex. In contrast, overexpression of COY1 inhibited the growth of mutant strains deficient in fusion activity at the Golgi (sed5-1 and sly1-ts). To map Coy1 protein interactions, co-immunoprecipitation experiments revealed an association with the Conserved Oliogmeric Golgi (COG) complex and with intra-Golgi SNARE proteins. These physical interactions are direct, as Coy1 was efficiently captured in vitro by Lobe A of the COG complex and the purified SNARE proteins Gos1, Sed5 and Sft1. Thus, our genetic, in vivo, and biochemical data indicate a role for Coy1 in regulating COG complex-dependent fusion of retrograde-directed COPI vesicles.

Endoproteolytic cleavage of TUG protein regulates GLUT4 glucose transporter translocation

To promote glucose uptake into fat and muscle cells, insulin causes the translocation of GLUT4 glucose transporters from intracellular vesicles to the cell surface. Previous data support a model in which TUG traps GLUT4-containing vesicles and tethers them intracellularly in unstimulated cells and in which insulin mobilizes this pool of vesicles by releasing this tether. Here we show that TUG undergoes site-specific endoproteolytic cleavage, which separates a GLUT4-binding, N-terminal region of TUG from a C-terminal region previously suggested to bind an intracellular anchor. Cleavage is accelerated by insulin stimulation in 3T3-L1 adipocytes and is highly dependent upon adipocyte differentiation. The N-terminal TUG cleavage product has properties of a novel 18-kDa ubiquitin-like modifier, which we call TUGUL. The C-terminal product is observed at the expected size of 42 kDa and also as a 54-kDa form that is released from membranes into the cytosol. In transfected cells, intact TUG links GLUT4 to PIST and also binds Golgin-160 through its C-terminal region. PIST is an effector of TC10α, a GTPase previously shown to transmit an insulin signal required for GLUT4 translocation, and we show using RNAi that TC10α is required for TUG proteolytic processing. Finally, we demonstrate that a cleavage-resistant form of TUG does not support highly insulin-responsive GLUT4 translocation or glucose uptake in 3T3-L1 adipocytes. Together with previous results, these data support a model whereby insulin stimulates TUG cleavage to liberate GLUT4 storage vesicles from the Golgi matrix, which promotes GLUT4 translocation to the cell surface and enhances glucose uptake.

New components of the Golgi matrix

The eukaryotic Golgi apparatus is characterized by a stack of flattened cisternae that are surrounded by transport vesicles. The organization and function of the Golgi require Golgi matrix proteins, including GRASPs and golgins, which exist primarily as fiber-like bridges between Golgi cisternae or between cisternae and vesicles. In this review, we highlight recent findings on Golgi matrix proteins, including their roles in maintaining the Golgi structure, vesicle tethering, and novel, unexpected functions. These new discoveries further our understanding of the molecular mechanisms that maintain the structure and the function of the Golgi, as well as its relationship with other cellular organelles such as the centrosome.

Role of vesicle tethering factors in the ER-Golgi membrane traffic

Tethers are a diverse group of loosely related proteins and protein complexes grouped into three families based on structural and functional similarities. A well-accepted role for tethering factors is the initial attachment of transport carriers to acceptor membranes prior to fusion. However, accumulating evidence indicates that tethers are more than static bridges. Tethers have been shown to interact with components of the fusion machinery and with components involved in vesicle formation. Tethers belonging to the three families act at the same stage of traffic, suggesting that they mediate distinct events during vesicle tethering. Thus, multiple tether-facilitated events are required to provide selectivity to vesicle fusion. In this review, we highlight findings that support this model.

The Golgin GMAP210/TRIP11 anchors IFT20 to the Golgi complex

Eukaryotic cells often use proteins localized to the ciliary membrane to monitor the extracellular environment. The mechanism by which proteins are sorted, specifically to this subdomain of the plasma membrane, is almost completely unknown. Previously, we showed that the IFT20 subunit of the intraflagellar transport particle is localized to the Golgi complex, in addition to the cilium and centrosome, and hypothesized that the Golgi pool of IFT20 plays a role in sorting proteins to the ciliary membrane. Here, we show that IFT20 is anchored to the Golgi complex by the golgin protein GMAP210/Trip11. Mice lacking GMAP210 die at birth with a pleiotropic phenotype that includes growth restriction, ventricular septal defects of the heart, omphalocele, and lung hypoplasia. Cells lacking GMAP210 have normal Golgi structure, but IFT20 is no longer localized to this organelle. GMAP210 is not absolutely required for ciliary assembly, but cilia on GMAP210 mutant cells are shorter than normal and have reduced amounts of the membrane protein polycystin-2 localized to them. This work suggests that GMAP210 and IFT20 function together at the Golgi in the sorting or transport of proteins destined for the ciliary membrane.

The primary cilium is a sensory organelle used by cells to monitor the extracellular environment. In mouse, severe defects in primary cilia lead to embryonic lethality while less severe defects cause a pleiotrophic phenotype that includes cystic kidney disease, retinal degeneration, obesity, and hydrocephaly, among others. The sensory functions of cilia rely on proteins localized to the ciliary membrane, which is continuous with the plasma membrane of the cell. Cells have the ability to specifically localize proteins to the ciliary membrane to the exclusion of the rest of the plasma membrane. Little is known about how this is accomplished. In prior work, we showed that the ciliary assembly protein IFT20 is localized to the Golgi complex, in addition to the cilium, and we proposed that it is involved in sorting or transport of membrane proteins to the cilium. In this work, we show that IFT20 is anchored to the Golgi complex by the golgin GMAP210. Mice defective in GMAP210 die at birth with lung and heart defects. Cells from these animals have ciliary defects, suggesting that IFT20 and GMAP210 function together at the Golgi complex in the trafficking of ciliary membrane proteins.

Yarrowia lipolytica vesicle-mediated protein transport pathways

BACKGROUND

Protein secretion is a universal cellular process involving vesicles which bud and fuse between organelles to bring proteins to their final destination. Vesicle budding is mediated by protein coats; vesicle targeting and fusion depend on Rab GTPase, tethering factors and SNARE complexes. The Génolevures II sequencing project made available entire genome sequences of four hemiascomycetous yeasts, Yarrowia lipolytica, Debaryomyces hansenii, Kluyveromyces lactis and Candida glabrata. Y. lipolytica is a dimorphic yeast and has good capacities to secrete proteins. The translocation of nascent protein through the endoplasmic reticulum membrane was well studied in Y. lipolytica and is largely co-translational as in the mammalian protein secretion pathway.

RESULTS

We identified S. cerevisiae proteins involved in vesicular secretion and these protein sequences were used for the BLAST searches against Génolevures protein database (Y. lipolytica, C. glabrata, K. lactis and D. hansenii). These proteins are well conserved between these yeasts and Saccharomyces cerevisiae. We note several specificities of Y. lipolytica which may be related to its good protein secretion capacities and to its dimorphic aspect. An expansion of the Y. lipolytica Rab protein family was observed with autoBLAST and the Rab2- and Rab4-related members were identified with BLAST against NCBI protein database. An expansion of this family is also found in filamentous fungi and may reflect the greater complexity of the Y. lipolytica secretion pathway. The Rab4p-related protein may play a role in membrane recycling as rab4 deleted strain shows a modification of colony morphology, dimorphic transition and permeability. Similarly, we find three copies of the gene (SSO) encoding the plasma membrane SNARE protein. Quantification of the percentages of proteins with the greatest homology between S. cerevisiae, Y. lipolytica and animal homologues involved in vesicular transport shows that 40% of Y. lipolytica proteins are closer to animal ones, whereas they are only 13% in the case of S. cerevisiae.

CONCLUSION

These results provide further support for the idea, previously noted about the endoplasmic reticulum translocation pathway, that Y. lipolytica is more representative of vesicular secretion of animals and other fungi than is S. cerevisiae.

A novel 165-kDa Golgin protein induced by brain ischemia and phosphorylated by Akt protects against apoptosis

A cDNA encoding a novel protein was cloned from ischemic rat brain and found to be homologous to testis Mea-2 Golgi-associated protein (Golga3). The sequence predicted a 165-kDa protein, and in vitro translated protein exhibited a molecular mass of 165-170 kDa. Because brain ischemia induced the mRNA, and the protein localized to the Golgi apparatus, this protein was designated Ischemia-Inducible Golgin Protein 165 (IIGP165). In HeLa cells, serum and glucose deprivation-induced caspase-dependent cleavage of the IIGP165 protein, after which the IIGP165 fragments translocated to the nucleus. The C-terminus of IIGP165, which contains a LXXLL motif, appears to function as a transcriptional co-regulator. Akt co-localizes with IIGP165 protein in the Golgi in vivo, and phosphorylates IIGP165 on serine residues 345 and 134. Though transfection of IIGP165 cDNA alone does not protect HeLa cells from serum deprivation or Brefeldin-A-triggered cell death, co-transfection of both Akt and IIGP165 cDNA or combined IIGP165-transfection with PDGF treatment significantly protects HeLa cells better than either treatment alone. These data show that Akt phosphorylation of IIGP165 protects against apoptotic cell death, and add to evidence that the Golgi apparatus also plays a role in regulating apoptosis.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

Characterization of the yeast amphiphysins Rvs161p and Rvs167p reveals roles for the Rvs heterodimer in vivo

We have used comprehensive synthetic lethal screens and biochemical assays to examine the biological role of the yeast amphiphysin homologues Rvs161p and Rvs167p, two proteins that play a role in regulation of the actin cytoskeleton, endocytosis, and sporulation. We found that unlike some forms of amphiphysin, Rvs161p-Rvs167p acts as an obligate heterodimer during vegetative growth and neither Rvs161p nor Rvs167p forms a homodimer in vivo. RVS161 and RVS167 have an identical set of 49 synthetic lethal interactions, revealing functions for the Rvs proteins in cell polarity, cell wall synthesis, and vesicle trafficking as well as a shared role in mating. Consistent with these roles, we show that the Rvs167p-Rvs161p heterodimer, like its amphiphysin homologues, can bind to phospholipid membranes in vitro, suggesting a role in vesicle formation and/or fusion. Our genetic screens also reveal that the interaction between Abp1p and the Rvs167p Src homology 3 (SH3) domain may be important under certain conditions, providing the first genetic evidence for a role for the SH3 domain of Rvs167p. Our studies implicate heterodimerization of amphiphysin family proteins in various functions related to cell polarity, cell integrity, and vesicle trafficking during vegetative growth and the mating response.

Holding it all together? Candidate proteins for the plant Golgi matrix

A combination of electron microscopy and fluorescence microscopy has provided us with a global picture of the structure of the plant Golgi apparatus. However, the components that shape this structure remain elusive. In other organisms, members of the golgin family of coiled-coil proteins are essential for Golgi structure and organisation. Putative Arabidopsis and rice homologues of some golgin family members can be identified using database searches. Likewise, the heterogeneous group of multi-subunit-tethering complexes is responsible for crucial transport steps that affect Golgi structure and cisternal organisation in animals and yeasts. The Arabidopsis genome harbours possible homologues for the majority of the subunits of these complexes, suggesting that they also operate in the plant kingdom.

Golgi tethering factors

Transport of cargo to, through and from the Golgi complex is mediated by vesicular carriers and transient tubular connections. In this review, we describe vesicle tethering events with the understanding that similar events occur during transport via larger structures. Tethering factors can be generally divided into a group of coiled-coil proteins and a group of multi-subunit complexes. Current evidence suggests that these factors function in a variety of membrane-membrane tethering events at the Golgi complex, interact with SNARE molecules, and are regulated by small GTPases of the Rab and Arl families.

Golgi positioning: are we looking at the right MAP?

One of the characteristics of the mammalian Golgi is its position adjacent to the nucleus. This characteristic is maintained through the action of the microtubule (MT) minus end–directed motor dynein and MT-associated proteins (MAPs). Recent findings suggest that GMAP-210, a member of the golgin family of proteins, may help to link Golgi membranes and vesicles with the MT cytoskeleton. However, there are good grounds to doubt that either GMAP-210 or its yeast homologue Rud3p is a MAP. Instead, they appear to function in vesicle trafficking events at the Golgi together with the GTPase ARF1 and a small membrane protein, Erv14. As such, the interesting question of how the Golgi interacts with MTs may well remain open to further investigation.

The GTPase Arf1p and the ER to Golgi cargo receptor Erv14p cooperate to recruit the golgin Rud3p to the cis-Golgi

Rud3p is a coiled-coil protein of the yeast cis-Golgi. We find that Rud3p is localized to the Golgi via a COOH-terminal domain that is distantly related to the GRIP domain that recruits several coiled-coil proteins to the trans-Golgi by binding the small Arf-like GTPase Arl1p. In contrast, Rud3p binds to the GTPase Arf1p via this COOH-terminal "GRIP-related Arf-binding" (GRAB) domain. Deletion of RUD3 is lethal in the absence of the Golgi GTPase Ypt6p, and a screen of other mutants showing a similar genetic interaction revealed that Golgi targeting of Rud3p also requires Erv14p, a cargo receptor that cycles between the endoplasmic reticulum and Golgi. The one human protein with a GRAB domain, GMAP-210 (CEV14/Trip11/Trip230), is known to be on the cis-Golgi, but the COOH-terminal region that contains the GRAB domain has been reported to bind to centrosomes and gamma-tubulin (Rios, R.M, A. Sanchis, A.M. Tassin, C. Fedriani, and M. Bornens. 2004. Cell. 118:323-335). In contrast, we find that this region binds to the Golgi in a GRAB domain-dependent manner, suggesting that GMAP-210 may not link the Golgi to gamma-tubulin and centrosomes.