The mechanism of Golgi segregation during mitosis is cell type-specific

Multifaceted control of E-cadherin dynamics by Adaptor Protein Complex 1 during epithelial morphogenesis

Intracellular trafficking regulates the distribution of transmembrane proteins including the key determinants of epithelial polarity and adhesion. The Adaptor Protein 1 (AP-1) complex is the key regulator of vesicle sorting, which binds many specific cargoes. We examined roles of the AP-1 complex in epithelial morphogenesis, using the Drosophila wing as a paradigm. We found that AP-1 knockdown leads to ectopic tissue folding, which is consistent with the observed defects in integrin targeting to the basal cell-extracellular matrix adhesion sites. This occurs concurrently with an integrin-independent induction of cell death, which counteracts elevated proliferation and prevents hyperplasia. We discovered a distinct pool of AP-1 that localizes at the subapical adherens junctions. Upon AP-1 knockdown, E-cadherin is hyperinternalized from these junctions and becomes enriched at the Golgi and recycling endosomes. We then provide evidence that E-cadherin hyperinternalization acts upstream of cell death in a potential tumor-suppressive mechanism. Simultaneously, cells compensate for elevated internalization of E-cadherin by increasing its expression to maintain cell-cell adhesion.

Mechanisms of membrane traffic in plant cells

The organelles of the secretory pathway are characterized by specific organization and function but they communicate in different ways with intense functional crosstalk. The best known membrane-bound transport carriers are known as protein-coated vesicles. Other traffic mechanisms, despite the intense investigations, still show incongruences. The review intends to provide a general view of the mechanisms involved in membrane traffic. We evidence that organelles' biogenesis involves mechanisms that actively operate during the entire cell cycle and the persistent interconnections between the Endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN) and endosomes, the vacuolar complex and the plasma membrane (PM) may be seen as a very dynamic membrane network in which vesicular traffic is part of a general maturation process.

Function of Golgi-centrosome proximity in RPE-1 cells

The close physical proximity between the Golgi and the centrosome is a unique feature of mammalian cells that has baffled scientists for years. Several knockdown and overexpression studies have linked the spatial relationship between these two organelles to the control of directional protein transport, directional migration, ciliogenesis and mitotic entry. However, most of these conditions have not only separated these two organelles, but also caused extensive fragmentation of the Golgi, making it difficult to dissect the specific contribution of Golgi-centrosome proximity. In this study, we present our results with stable retinal pigment epithelial (RPE-1) cell lines in which GM130 was knocked out using a CRISPR/Cas9 approach. While Golgi and centrosome organization appeared mostly intact in cells lacking GM130, there was a clear separation of these organelles from each other. We show that GM130 may control Golgi-centrosome proximity by anchoring AKAP450 to the Golgi. We also provide evidence that the physical proximity between these two organelles is dispensable for protein transport, cell migration, and ciliogenesis. These results suggest that Golgi-centrosome proximity per se is not necessary for the normal function of RPE-1 cells.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

Ultrastructural analysis of mitotic Drosophila S2 cells identifies distinctive microtubule and intracellular membrane behaviors

BACKGROUND

S2 cells are one of the most widely used Drosophila melanogaster cell lines. A series of studies has shown that they are particularly suitable for RNAi-based screens aimed at the dissection of cellular pathways, including those controlling cell shape and motility, cell metabolism, and host-pathogen interactions. In addition, RNAi in S2 cells has been successfully used to identify many new mitotic genes that are conserved in the higher eukaryotes, and for the analysis of several aspects of the mitotic process. However, no detailed and complete description of S2 cell mitosis at the ultrastructural level has been done. Here, we provide a detailed characterization of all phases of S2 cell mitosis visualized by transmission electron microscopy (TEM).

RESULTS

We analyzed by TEM a random sample of 144 cells undergoing mitosis, focusing on intracellular membrane and microtubule (MT) behaviors. This unbiased approach provided a comprehensive ultrastructural view of the dividing cells, and allowed us to discover that S2 cells exhibit a previously uncharacterized behavior of intracellular membranes, involving the formation of a quadruple nuclear membrane in early prometaphase and its disassembly during late prometaphase. After nuclear envelope disassembly, the mitotic apparatus becomes encased by a discontinuous network of endoplasmic reticulum membranes, which associate with mitochondria, presumably to prevent their diffusion into the spindle area. We also observed a peculiar metaphase spindle organization. We found that kinetochores with attached k-fibers are almost invariably associated with lateral MT bundles that can be either interpolar bundles or k-fibers connected to a different kinetochore. This spindle organization is likely to favor chromosome alignment at metaphase and subsequent segregation during anaphase.

CONCLUSIONS

We discovered several previously unknown features of membrane and MT organization during S2 cell mitosis. The genetic determinants of these mitotic features can now be investigated, for instance by using an RNAi-based approach, which is particularly easy and efficient in S2 cells.

Differential expression, distinct localization and opposite effect on Golgi structure and cell differentiation by a novel splice variant of human PRMT5

Alternative splicing contributes greatly to the proteomic diversity of metazoans. Protein arginine methyltransferase 5 (PRMT5) methylates arginines of Golgi components and other factors exerting diverse effects on cell growth/differentiation, but the underlying molecular basis for its subcellular distribution and diverse roles has not been fully understood. Here we show the detailed properties of an evolutionarily emerged splice variant of human PRMT5 (PRMT5S) that is distinct from the original isoform (PRMT5L). The isoforms are differentially expressed among mammalian cells and tissues. The PRMT5S is distributed all over the cell but PRMT5L mainly colocalizes with Giantin, a Golgi marker. PRMT5 knockdown led to an enlarged Giantin pattern, which was prevented by the expression of either isoform. Rescuing PRMT5S also increased the percentage of cells with an interphase Giantin pattern compacted at one end of the nucleus, consistent with its cell cycle-arresting effect, while rescuing PRMT5L increased that of the mitotic Giantin patterns of dynamically fragmented structures. Moreover, the isoforms are differentially expressed during neuronal or dendritic cell differentiation, and their ectopic expression showed an opposite effect on dendritic cell differentiation. Furthermore, besides their differential regulation of gene expression, both isoforms also similarly regulate over a thousand genes particularly those involved in apoptosis and differentiation. Taking these properties together, we propose that their differential expression and subcellular localization contribute to spatial and temporal regulation of arginine methylation and gene expression to exert different effects. The novel PRMT5S likely contributes to the observed diverse effects of PRMT5 in cells.

Formation and maintenance of the Golgi apparatus in plant cells

The Golgi apparatus plays essential roles in intracellular trafficking, protein and lipid modification, and polysaccharide synthesis in eukaryotic cells. It is well known for its unique stacked structure, which is conserved among most eukaryotes. However, the mechanisms of biogenesis and maintenance of the structure, which are deeply related to ER-Golgi and intra-Golgi transport systems, have long been mysterious. Now having extremely powerful microscopic technologies developed for live-cell imaging, the plant Golgi apparatus provides an ideal system to resolve the question. The plant Golgi apparatus has unique features that are not conserved in other kingdoms, which will also give new insights into the Golgi functions in plant life. In this review, we will summarize the features of the plant Golgi apparatus and transport mechanisms around it, with a focus on recent advances in Golgi biogenesis by live imaging of plants cells.

The Drosophila MAST kinase Drop out is required to initiate membrane compartmentalisation during cellularisation and regulates dynein-based transport

Cellularisation of the Drosophila syncytial blastoderm embryo into the polarised blastoderm epithelium provides an excellent model with which to determine how cortical plasma membrane asymmetry is generated during development. Many components of the molecular machinery driving cellularisation have been identified, but cell signalling events acting at the onset of membrane asymmetry are poorly understood. Here we show that mutations in drop out (dop) disturb the segregation of membrane cortical compartments and the clustering of E-cadherin into basal adherens junctions in early cellularisation. dop is required for normal furrow formation and controls the tight localisation of furrow canal proteins and the formation of F-actin foci at the incipient furrows. We show that dop encodes the single Drosophila homologue of microtubule-associated Ser/Thr (MAST) kinases. dop interacts genetically with components of the dynein/dynactin complex and promotes dynein-dependent transport in the embryo. Loss of dop function reduces phosphorylation of Dynein intermediate chain, suggesting that dop is involved in regulating cytoplasmic dynein activity through direct or indirect mechanisms. These data suggest that Dop impinges upon the initiation of furrow formation through developmental regulation of cytoplasmic dynein.

The Arf family G protein Arl1 is required for secretory granule biogenesis in Drosophila

The small G protein Arf like 1 (Arl1) is found at the Golgi complex, and its GTP-bound form recruits several effectors to the Golgi including GRIP-domain-containing coiled-coil proteins, and the Arf1 exchange factors Big1 and Big2. To investigate the role of Arl1, we have characterised a loss-of-function mutant of the Drosophila Arl1 orthologue. The gene is essential, and examination of clones of cells lacking Arl1 shows that it is required for recruitment of three of the four GRIP domain golgins to the Golgi, with Drosophila GCC185 being less dependent on Arl1. At a functional level, Arl1 is essential for formation of secretory granules in the larval salivary gland. When Arl1 is missing, Golgi are still present but there is a dispersal of adaptor protein 1 (AP-1), a clathrin adaptor that requires Arf1 for its membrane recruitment and which is known to be required for secretory granule biogenesis. Arl1 does not appear to be required for AP-1 recruitment in all tissues, suggesting that it is crucially required to enhance Arf1 activation at the trans-Golgi in particular tissues.

The amyotrophic lateral sclerosis 8 protein, VAP, is required for ER protein quality control

A familial form of Amyotrophic lateral sclerosis (ALS8) is caused by a point mutation (P56S) in the vesicle-associated membrane protein associated protein B (VapB). Human VapB and Drosophila Vap-33-1 (Vap) are homologous type II transmembrane proteins that are localized to the ER. However, the precise consequences of the defects associated with the P56S mutation in the endoplasmic reticulum (ER) and its role in the pathology of ALS are not well understood. Here we show that Vap is required for ER protein quality control (ERQC). Loss of Vap in flies shows various ERQC associated defects, including protein accumulation, ER expansion, and ER stress. We also show that wild type Vap, but not the ALS8 mutant Vap, interacts with a lipid-binding protein, Oxysterol binding protein (Osbp), and that Vap is required for the proper localization of Osbp to the ER. Restoring the expression of Osbp in the ER suppresses the defects associated with loss of Vap and the ALS8 mutant Vap. Hence, we propose that the ALS8 mutation impairs the interaction of Vap with Osbp, resulting in hypomorphic defects that might contribute to the pathology of ALS8.

Distribution and morphological changes of the Golgi apparatus during Drosophila spermatogenesis

In spermatogenesis, the Golgi apparatus is important for the formation of the acrosome, which is a sperm-specific organelle essential for fertilization. Comprehensive examinations of the spatiotemporal distribution and morphological characterizations of the Golgi in various cells during spermatogenesis are necessary for functional analyses and mutant screenings in the model eukaryote Drosophila. Here, we examined the distribution and morphology of the Golgi during Drosophila spermatogenesis with immunofluorescence and electron microscopy. In pre-meiotic germ cells, the Golgi apparatuses were distributed evenly in the cytoplasm. In contrast, they were located exclusively in two regions near the poles during the meiotic metaphase, where they were segregated prior to the chromosomes. In cells in anaphase to telophase, the Golgi were predominantly left behind in the equatorial region between the separating daughter nuclei. After completion of meiosis, the dispersed Golgi were assembled at the apical side of the spermatid nucleus to form the acrosome. Further investigation of the Golgi distribution in β2-tubulin mutants showed aberrant and uneven distributions of the Golgi among sister cells in the meiotic spermatocytes and in the post-meiotic spermatids. At the ultrastructural level, the Golgi apparatus in pre-meiotic spermatocytes comprised a pair of stacks. The two stacks were situated adjacent to each other, as if they had duplicated before entering into meiotic division. These results highlight the dynamic nature of the Golgi during spermatogenesis and provide a framework for analyzing the correlations between the dynamics of the Golgi and its function in sperm development.

Polar substitutions in helix 3 of the prion protein produce transmembrane isoforms that disturb vesicle trafficking

Prion diseases encompass a diverse group of neurodegenerative conditions characterized by the accumulation of misfolded prion protein (PrP) isoforms. Other conformational variants of PrP have also been proposed to contribute to neurotoxicity in prion diseases, including misfolded intermediates as well as cytosolic and transmembrane isoforms. To better understand PrP neurotoxicity, we analyzed the role of two highly conserved methionines in helix 3 on PrP biogenesis, folding and pathogenesis. Expression of the PrP-M205S and -M205,212S mutants in Drosophila led to hyperglycosylation, intracellular accumulation and widespread conformational changes due to failure of oxidative folding. Surprisingly, PrP-M205S and -M205,212S acquired a transmembrane topology (Ctm) previously linked to mutations in the signal peptide (SP) and the transmembrane domain (TMD). PrP-M205,212S also disrupted the accumulation of key neurodevelopmental proteins in lipid rafts, resulting in shortened axonal projections. These results uncover a new role for the hydrophobic domain in promoting oxidative folding and preventing the formation of neurotoxic Ctm PrP, mechanisms that may be relevant in the pathogenesis of both inherited and sporadic prion diseases.

The Drosophila RZZ complex - roles in membrane trafficking and cytokinesis

The Zw10 protein, in the context of the conserved Rod-Zwilch-Zw10 (RZZ) complex, is a kinetochore component required for proper activity of the spindle assembly checkpoint in both Drosophila and mammals. In mammalian and yeast cells, the Zw10 homologues, together with the conserved RINT1/Tip20p and NAG/Sec39p proteins, form a second complex involved in vesicle transport between Golgi and ER. However, it is currently unknown whether Zw10 and the NAG family member Rod are also involved in Drosophila membrane trafficking. Here we show that Zw10 is enriched at both the Golgi stacks and the ER of Drosophila spermatocytes. Rod is concentrated at the Golgi but not at the ER, whereas Zwilch does not accumulate in any membrane compartment. Mutations in zw10 and RNAi against the Drosophila homologue of RINT1 (rint1) cause strong defects in Golgi morphology and reduce the number of Golgi stacks. Mutations in rod also affect Golgi morphology, whereas zwilch mutants do not exhibit gross Golgi defects. Loss of either Zw10 or Rint1 results in frequent failures of spermatocyte cytokinesis, whereas Rod or Zwilch are not required for this process. Spermatocytes lacking zw10 or rint1 function assemble regular central spindles and acto-myosin rings, but furrow ingression halts prematurely due to defective plasma membrane addition. Collectively, our results suggest that Zw10 and Rint1 cooperate in the ER-Golgi trafficking and in plasma membrane formation during spermatocyte cytokinesis. Our findings further suggest that Rod plays a Golgi-related function that is not required for spermatocyte cytokinesis.

The Drosophila Sec7 domain guanine nucleotide exchange factor protein Gartenzwerg localizes at the cis-Golgi and is essential for epithelial tube expansion

Protein trafficking through the secretory pathway plays a key role in epithelial organ development and function. The expansion of tracheal tubes in Drosophila depends on trafficking of coatomer protein complex I (COPI)-coated vesicles between the Golgi complex and the endoplasmic reticulum (ER). However, it is not clear how this pathway is regulated. Here we describe an essential function of the Sec7 domain guanine nucleotide exchange factor (GEF) gartenzwerg (garz) in epithelial tube morphogenesis and protein secretion. garz is essential for the recruitment of COPI components and for normal Golgi organization. A GFP-Garz fusion protein is distributed in the cytoplasm and accumulates at the cis-Golgi. Localization to the Golgi requires the C-terminal part of Garz. Conversely, blocking the GDP-GTP nucleotide exchange reaction leads to constitutive Golgi localization, suggesting that Garz cycles in a GEF-activity-dependent manner between cytoplasmic and Golgi-membrane-localized pools. The related human ARF-GEF protein GBF1 can substitute for garz function in Drosophila tracheal cells, indicating that the relevant functions of these proteins are conserved. We show that garz interacts genetically with the ARF1 homolog ARF79F and with the ARF1-GAP homolog Gap69C, thus placing garz in a regulatory circuit that controls COPI trafficking in Drosophila. Interestingly, overexpression of garz causes accumulation of secreted proteins in the ER, suggesting that excessive garz activity leads to increased retrograde trafficking. Thus, garz might regulate epithelial tube morphogenesis and secretion by controlling the rate of trafficking of COPI vesicles.

Cisterna-specific localization of glycosylation-related proteins to the Golgi apparatus

The Golgi apparatus is an intracellular organelle playing central roles in post-translational modification and in the secretion of membrane and secretory proteins. These proteins are synthesized in the endoplasmic reticulum (ER) and transported to the cis-, medial-and trans-cisternae of the Golgi. While trafficking through the Golgi, proteins are sequentially modified with glycan moieties by different glycosyltransferases. Therefore, it is important to analyze the glycosylation function of the Golgi at the level of cisternae. Markers widely used for cis-, medial- and trans-cisternae/trans Golgi network (TGN) in Drosophila are GM130, 120 kDa and Syntaxin16 (Syx16); however the anti-120 kDa antibody is no longer available. In the present study, Drosophila Golgi complex-localized glycoprotein-1 (dGLG1) was identified as an antigen recognized by the anti-120 kDa antibody. A monoclonal anti-dGLG1 antibody suitable for immunohistochemistry was raised in rat. Using these markers, the localization of glycosyltransferases and nucleotide-sugar transporters (NSTs) was studied at the cisternal level. Results showed that glycosyltransferases and NSTs involved in the same sugar modification are localized to the same cisternae. Furthermore, valuable functional information was obtained on the localization of novel NSTs with as yet incompletely characterized biochemical properties.

Rich regulates target specificity of photoreceptor cells and N-cadherin trafficking in the Drosophila visual system via Rab6

Neurons establish specific synaptic connections with their targets, a process that is highly regulated. Numerous cell adhesion molecules have been implicated in target recognition, but how these proteins are precisely trafficked and targeted is poorly understood. To identify components that affect synaptic specificity, we carried out a forward genetic screen in the Drosophila eye. We identified a gene, named ric1 homologue (rich), whose loss leads to synaptic specificity defects. Loss of rich leads to reduction of N-Cadherin in the photoreceptor cell synapses but not of other proteins implicated in target recognition, including Sec15, DLAR, Jelly belly, and PTP69D. The Rich protein binds to Rab6, and Rab6 mutants display very similar phenotypes as the rich mutants. The active form of Rab6 strongly suppresses the rich synaptic specificity defect, indicating that Rab6 is regulated by Rich. We propose that Rich activates Rab6 to regulate N-Cadherin trafficking and affects synaptic specificity.

Liquid facets-related (lqfR) is required for egg chamber morphogenesis during Drosophila oogenesis

Clathrin interactor 1 [CLINT1] (also called enthoprotin/EpsinR) is an Epsin N-terminal homology (ENTH) domain-containing adaptor protein that functions in anterograde and retrograde clathrin-mediated trafficking between the trans-Golgi network and the endosome. Removal of both Saccharomyces cerevisiae homologs, Ent3p and Ent5p, result in yeast that are viable, but that display a cold-sensitive growth phenotype and mistrafficking of various vacuolar proteins. Similarly, either knock-down or overexpression of vertebrate CLINT1 in cell culture causes mistrafficking of proteins. Here, we have characterized Drosophila CLINT1, liquid-facets Related (lqfR). LqfR is ubiquitously expressed throughout development and is localized to the Golgi and endosome. Strong hypomorphic mutants generated by imprecise P-element excision exhibit extra macrochaetae, rough eyes and are female sterile. Although essentially no eggs are laid, the ovaries do contain late-stage egg chambers that exhibit abnormal morphology. Germline clones reveal that LqfR expression in the somatic follicle cells is sufficient to rescue the oogenesis defects. Clones of mutant lqfR follicle cells have a decreased cell size consistent with a downregulation of Akt1. We find that while total Akt1 levels are increased there is also a significant decrease in activated phosphorylated Akt1. Taken together, these results show that LqfR function is required to regulate follicle cell size and signaling during Drosophila oogenesis.

Drosophila spoonbill encodes a dual-specificity A-kinase anchor protein essential for oogenesis

spoonbill is a Drosophila female-sterile mutation, which interferes with normal egg patterning during oogenesis. Previous analyzes linked the mutation to a number of seemingly unrelated pathways, including GRK/EGFR and DPP, two major pathways essential for Drosophila and vertebrate development. Further work suggested that spoonbill may also function in actin polymerization and border-cell migration. Here we describe the molecular cloning of the spoonbill gene and characterize new mutant alleles, further demonstrating that spoonbill's function is essential during oogenesis. We found spoonbill to be allelic to CG3249 (also known as yu), which encodes the only known dual-specificity A-kinase anchor protein in Drosophila. Our data indicate that similar to mammalian AKAPs, Spoonbill protein contains a number of potential kinase and phosphatase binding motifs, and is targeted, in the ovary, to mitochondria and Golgi. Finally, we address some of spoonbill's mutant phenotypes from the perspective of the published data on the AKAP protein family.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

Wolbachia bacteria reside in host Golgi-related vesicles whose position is regulated by polarity proteins

Wolbachia pipientis are intracellular symbiotic bacteria extremely common in various organisms including Drosophila melanogaster, and are known for their ability to induce changes in host reproduction. These bacteria are present in astral microtubule-associated vesicular structures in host cytoplasm, but little is known about the identity of these vesicles. We report here that Wolbachia are restricted only to a group of Golgi-related vesicles concentrated near the site of membrane biogenesis and minus-ends of microtubules. The Wolbachia vesicles were significantly mislocalized in mutant embryos defective in cell/planar polarity genes suggesting that cell/tissue polarity genes are required for apical localization of these Golgi-related vesicles. Furthermore, two of the polarity proteins, Van Gogh/Strabismus and Scribble, appeared to be present in these Golgi-related vesicles. Thus, establishment of polarity may be closely linked to the precise insertion of Golgi vesicles into the new membrane addition site.

Golgi positioning

The Golgi apparatus in mammalian cells is positioned near the centrosome-based microtubule-organizing center (Fig. 1). Secretory cargo moves inward in membrane carriers for delivery to Golgi membranes in which it is processed and packaged for transport outward to the plasma membrane. Cytoplasmic dynein motor proteins (herein termed dynein) primarily mediate inward cargo carrier movement and Golgi positioning. These motors move along microtubules toward microtubule minus-ends embedded in centrosomes. Centripetal motility is controlled by a host of regulators whose precise functions remain to be determined. Significantly, a specific Golgi receptor for dynein has not been identified. This has impaired progress toward elucidation of membrane-motor-microtubule attachment in the periphery and, after inward movement, recycling of the motor for another round. Pericentrosomal positioning of the Golgi apparatus is dynamic. It is regulated during critical cellular processes such as mitosis, differentiation, cell polarization, and cell migration. Positioning is also important as it aligns the Golgi along an axis of cell polarity. In certain cell types, this promotes secretion directed to the proximal plasma membrane domain thereby maintaining specializations critical for diverse processes including wound healing, immunological synapse formation, and axon determination.

Unraveling the Golgi ribbon

The Golgi apparatus lies at the heart of the secretory pathway where it receives, modifies and sorts protein cargo to the proper intracellular or extracellular location. Although this secretory function is highly conserved throughout the eukaryotic kingdom, the structure of the Golgi complex is arranged very differently among species. In particular, Golgi membranes in vertebrate cells are integrated into a single compact entity termed the Golgi ribbon that is normally localized in the perinuclear area and in close vicinity to the centrosomes. This organization poses a challenge for cell division when the single Golgi ribbon needs to be partitioned into the two daughter cells. To ensure faithful inheritance in the progeny, the Golgi ribbon is divided in three consecutive steps in mitosis, namely disassembly, partitioning and reassembly. However, the structure of the Golgi ribbon is only present in higher animals and Golgi disassembly during mitosis is not ubiquitous in all organisms. Therefore, there must be unique reasons to build up the Golgi in this particular conformation and to preserve it over generations. In this review, we first highlight the diversity of the Golgi architecture in different organisms and revisit the concept of the Golgi ribbon. Following on, we discuss why the ribbon is needed and how it forms in vertebrate cells. Lastly, we conclude with likely purposes of mitotic ribbon disassembly and further propose mechanisms by which it regulates mitosis.

Sugar-free frosting, a homolog of SAD kinase, drives neural-specific glycan expression in the Drosophila embryo

Precise glycan structures on specific glycoproteins impart functionalities essential for neural development. However, mechanisms controlling embryonic neural-specific glycosylation are unknown. A genetic screen for relevant mutations in Drosophila generated the sugar-free frosting (sff) mutant that reveals a new function for protein kinases in regulating substrate flux through specific Golgi processing pathways. Sff is the Drosophila homolog of SAD kinase, which regulates synaptic vesicle tethering and neuronal polarity in nematodes and vertebrates. Our Drosophila sff mutant phenotype has features in common with SAD kinase mutant phenotypes in these other organisms, but we detect altered neural glycosylation well before the initiation of embryonic synaptogenesis. Characterization of Golgi compartmentation markers indicates altered colocalization that is consistent with the detected shift in glycan complexity in sff mutant embryos. Therefore, in analogy to synaptic vesicle tethering, we propose that Sff regulates vesicle tethering at Golgi membranes in the developing Drosophila embryo. Furthermore, neuronal sff expression is dependent on transcellular signaling through a non-neural toll-like receptor, linking neural-specific glycan expression to a kinase activity that is induced in response to environmental cues.

The Golgi and the centrosome: building a functional partnership

The mammalian Golgi apparatus is characterized by a ribbon-like organization adjacent to the centrosome during interphase and extensive fragmentation and dispersal away from the centrosome during mitosis. It is not clear whether this dynamic association between the Golgi and centrosome is of functional significance. We discuss recent findings indicating that the Golgi–centrosome relationship may be important for directional protein transport and centrosome positioning, which are both required for cell polarization. We also summarize our current knowledge of the link between Golgi organization and cell cycle progression.

The Golgi apparatus: lessons from Drosophila

Historically, Drosophila has been a model organism for studying molecular and developmental biology leading to many important discoveries in this field. More recently, the fruit fly has started to be used to address cell biology issues including studies of the secretory pathway, and more specifically on the functional integrity of the Golgi apparatus. A number of advances have been made that are reviewed below. Furthermore, with the development of RNAi technology, Drosophila tissue culture cells have been used to perform genome-wide screens addressing similar issues. Last, the Golgi function has been involved in specific developmental processes, thus shedding new light on the functions of a number of Golgi proteins.

Homotypic fusion of ER membranes requires the dynamin-like GTPase atlastin

Establishment and maintenance of proper architecture is essential for endoplasmic reticulum (ER) function. Homotypic membrane fusion is required for ER biogenesis and maintenance, and has been shown to depend on GTP hydrolysis. Here we demonstrate that Drosophila Atlastin--the fly homologue of the mammalian GTPase atlastin 1 involved in hereditary spastic paraplegia--localizes on ER membranes and that its loss causes ER fragmentation. Drosophila Atlastin embedded in distinct membranes has the ability to form trans-oligomeric complexes and its overexpression induces enlargement of ER profiles, consistent with excessive fusion of ER membranes. In vitro experiments confirm that Atlastin autonomously drives membrane fusion in a GTP-dependent fashion. In contrast, GTPase-deficient Atlastin is inactive, unable to form trans-oligomeric complexes owing to failure to self-associate, and incapable of promoting fusion in vitro. These results demonstrate that Atlastin mediates membrane tethering and fusion and strongly suggest that it is the GTPase activity that is required for ER homotypic fusion.

Drosophila liquid facets-Related encodes Golgi epsin and is an essential gene required for cell proliferation, growth, and patterning

Epsin and epsin-Related (epsinR) are multi-modular proteins that stimulate clathrin-coated vesicle formation. Epsin promotes endocytosis at the plasma membrane, and epsinR functions at the Golgi and early endosomes for trans-Golgi network/endosome vesicle trafficking. In Drosophila, endocytic epsin is known as Liquid facets, and it is essential specifically for Notch signaling. Here, by generating and analyzing loss-of-function mutants in the liquid facets-Related (lqfR) gene of Drosophila, we investigated the function of Golgi epsin in a multicellular context. We found that LqfR is indeed a Golgi protein, and that like liquid facets, lqfR is essential for Drosophila viability. In addition, primarily by analyzing mutant eye discs, we found that lqfR is required for cell proliferation, insulin-independent cell growth, and cell patterning, consistent with a role in one or several signaling pathways. Epsins in all organisms share an ENTH (epsin N-terminal homology) domain, which binds phosphoinositides enriched at the plasma membrane or the Golgi membrane. The epsinR ENTH domain is also the recognition element for particular cargos. By generating wild-type and mutant lqfR transgenes, we found that all apparent LqfR functions are independent of its ENTH domain. These results suggest that LqfR transports specific cargo critical to one or more signaling pathways, and lays the foundation for identifying those proteins.

Timing cell-fate determination during asymmetric cell divisions

From invertebrates to mammals, cell-cycle progression during an asymmetric cell division is accompanied by precisely timed redistribution of cell-fate determinants so that they segregate asymmetrically to enable the two daughter cells to choose different fates. Interestingly, studies on how cell fates are specified in such divisions reveal that the same fate determinants can be reiteratively used to specify a variety of cell types through multiple rounds of cell divisions or to exert seemingly contradictory effects on cell proliferation and differentiation. Here I summarize the molecular mechanisms governing asymmetric cell division and review recent findings pointing to a novel mechanism for coupling intracellular signaling and cell-cycle progression. This mechanism uses changes in the morphology, subcellular distribution, and molecular composition of cellular organelles like the Golgi apparatus and centrosomes, which not only accompany the progression of cell cycle to activate but also temporally constrain the activity of fate determinants during asymmetric cell divisions.

A multicomponent assembly pathway contributes to the formation of acentrosomal microtubule arrays in interphase Drosophila cells

In animal cells, centrosomes nucleate microtubules that form polarized arrays to organize the cytoplasm. Drosophila presents an interesting paradox however, as centrosome-deficient mutant animals develop into viable adults. To understand this discrepancy, we analyzed behaviors of centrosomes and microtubules in Drosophila cells, in culture and in vivo, using a combination of live-cell imaging, electron microscopy, and RNAi. The canonical model of the cycle of centrosome function in animal cells states that centrosomes act as microtubule-organizing centers throughout the cell cycle. Unexpectedly, we found that many Drosophila cell-types display an altered cycle, in which functional centrosomes are only present during cell division. On mitotic exit, centrosomes disassemble producing interphase cells containing centrioles that lack microtubule-nucleating activity. Furthermore, steady-state interphase microtubule levels are not changed by codepleting both gamma-tubulins. However, gamma-tubulin RNAi delays microtubule regrowth after depolymerization, suggesting that it may function partially redundantly with another pathway. Therefore, we examined additional microtubule nucleating factors and found that Mini-spindles, CLIP-190, EB1, or dynein RNAi also delayed microtubule regrowth; surprisingly, this was not further prolonged when we codepleted gamma-tubulins. Taken together, these results modify our view of the cycle of centrosome function and reveal a multi-component acentrosomal microtubule assembly pathway to establish interphase microtubule arrays in Drosophila.

The endocytic pathway acts downstream of Oskar in Drosophila germ plasm assembly

Cell fate is often determined by the intracellular localization of RNAs and proteins. In Drosophila oocytes, oskar (osk) RNA localization and the subsequent Osk synthesis at the posterior pole direct the assembly of the pole plasm, where factors for the germline and abdomen formation accumulate. osk RNA produces two isoforms, long and short Osk, which have distinct functions in pole plasm assembly. Short Osk recruits downstream components of the pole plasm, whose anchoring to the posterior cortex requires long Osk. The anchoring of pole plasm components also requires actin cytoskeleton, and Osk promotes long F-actin projections in the oocyte posterior cytoplasm. However, the mechanism by which Osk mediates F-actin reorganization remains elusive. Furthermore, although long Osk is known to associate with endosomes under immuno-electron microscopy, it was not known whether this association is functionally significant. Here we show that Rabenosyn-5 (Rbsn-5), a Rab5 effector protein required for the early endocytic pathway, is crucial for pole plasm assembly. rbsn-5(-) oocytes fail to maintain microtubule polarity, which secondarily disrupts osk RNA localization. Nevertheless, anteriorly misexpressed Osk, particularly long Osk, recruits endosomal proteins, including Rbsn-5, and stimulates endocytosis. In oocytes lacking rbsn-5, the ectopic Osk induces aberrant F-actin aggregates, which diffuse into the cytoplasm along with pole plasm components. We propose that Osk stimulates endosomal cycling, which in turn promotes F-actin reorganization to anchor the pole plasm components to the oocyte cortex.

Wingless secretion promotes and requires retromer-dependent cycling of Wntless

Wnt ligands are lipid-modified, secreted glycoproteins that control multiple steps during embryogenesis and adult-tissue homeostasis. Little is known about the mechanisms underlying Wnt secretion. Recently, Wntless (Wls/Evi/Srt) was identified as a conserved multi-pass transmembrane protein whose function seems to be dedicated to promoting the release of Wnts. Here, we describe Wls accumulation in the Golgi apparatus of Wnt/Wingless (Wg)-producing cells in Drosophila, and show that this localization is essential for Wg secretion. Moreover, Wls localization and levels critically depend on retromer, a conserved protein complex that mediates endosome-to-Golgi protein trafficking in yeast. In the absence of the retromer components Dvps35 or Dvps26, but in presence of Wg, Wls is degraded and Wg secretion impaired. Our results indicate that Wg, clathrin-mediated endocytosis and retromer sustain a Wls traffic loop from the Golgi to the plasma membrane and back to the Golgi, thereby enabling Wls to direct Wnt secretion.

Drosophila huntingtin-interacting protein 14 is a presynaptic protein required for photoreceptor synaptic transmission and expression of the palmitoylated proteins synaptosome-associated protein 25 and cysteine string protein

Palmitoylation affects the trafficking, stability, aggregation, and/or functional activity of a substantial number of neuronal proteins. We identified mutations in dHIP14, the Drosophila homolog of the human palmitoyl transferase, Huntingtin-interacting protein 14 (HIP14). HIP14 was previously reported to localize primarily to Golgi and to palmitoylate the neuronal proteins synaptosome-associated protein 25 (SNAP-25), PSD-95 (postsynaptic density-95), GAD65, Synaptotagmin, and Huntingtin in mammalian neurons. We find dHIP14 to be an essential maternal effect gene required for photoreceptor synaptic transmission and for proper in vivo expression of the palmitoylated presynaptic proteins SNAP-25 and cysteine string protein. In non-neuronal cells in the fly, dHIP14 protein is found in Golgi. However, in fly neurons, we find dHIP14 primarily in presynaptic terminals, something we also observe with HIP14. In mammalian neurons, we also find a significant fraction of HIP14 colocalizing with a synaptic vesicle marker. Based on localization of the palmitoyl transferase HIP14 within the presynaptic nerve terminal, we propose palmitoylation as a possible mechanism that may be operating to rapidly regulate synaptic efficacy.

The Golgi-Associated Protein GRASP Is Required for Unconventional Protein Secretion during Development

During Dictyostelium development, prespore cells secrete acyl-CoA binding protein (AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and, thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell surface). GRASP is not required for conventional protein secretion, growth, and the viability of vegetative cells. Our findings reveal a physiological role of GRASP and provide a means to understand unconventional secretion and its role in development.

The golgi comprises a paired stack that is separated at G2 by modulation of the actin cytoskeleton through Abi and Scar/WAVE

During the cell cycle, the Golgi, like other organelles, has to be duplicated in mass and number to ensure its correct segregation between the two daughter cells. It remains unclear, however, when and how this occurs. Here we show that in Drosophila S2 cells, the Golgi likely duplicates in mass to form a paired structure during G1/S phase and remains so until G2 when the two stacks separate, ready for entry into mitosis. We show that pairing requires an intact actin cytoskeleton which in turn depends on Abi/Scar but not WASP. This actin-dependent pairing is not limited to flies but also occurs in mammalian cells. We further show that preventing the Golgi stack separation at G2 blocks entry into mitosis, suggesting that this paired organization is part of the mitotic checkpoint, similar to what has been proposed in mammalian cells.

Protein kinase D regulates several aspects of development in Drosophila melanogaster

Background

Protein Kinase D (PKD) is an effector of diacylglycerol-regulated signaling pathways. Three isoforms are known in mammals that have been linked to diverse cellular functions including regulation of cell proliferation, differentiation, motility and secretory transport from the trans-Golgi network to the plasma membrane. In Drosophila, there is a single PKD orthologue, whose broad expression implicates a more general role in development.

Results

We have employed tissue specific overexpression of various PKD variants as well as tissue specific RNAi, in order to investigate the function of the PKD gene in Drosophila. Apart from a wild type (WT), a kinase dead (kd) and constitutively active (SE) Drosophila PKD variant, we also analyzed two human isoforms hPKD2 and hPKD3 for their capacity to substitute PKD activity in the fly. Overexpression of either WT or kd-PKD variants affected primarily wing vein development. However, overexpression of SE-PKD and PKD RNAi was deleterious. We observed tissue loss, wing defects and degeneration of the retina. The latter phenotype conforms to a role of PKD in the regulation of cytoskeletal dynamics. Strongest phenotypes were larval to pupal lethality. RNAi induced phenotypes could be rescued by a concurrent overexpression of Drosophila wild type PKD or either human isoform hPKD2 and hPKD3.

Conclusion

Our data confirm the hypothesis that Drosophila PKD is a multifunctional kinase involved in diverse processes such as regulation of the cytoskeleton, cell proliferation and death as well as differentiation of various fly tissues.

The Mammalian Golgi Regulates Numb Signaling in Asymmetric Cell Division by Releasing ACBD3 during Mitosis

Mammalian neural progenitor cells divide asymmetrically to self-renew and produce a neuron by segregating cytosolic Numb proteins primarily to one daughter cell. Numb signaling specifies progenitor over neuronal fates but, paradoxically, also promotes neuronal differentiation. Here we report that ACBD3 is a Numb partner in cell-fate specification. ACBD3 and Numb proteins interact through an essential Numb domain, and the respective loss- and gain-of-function mutant mice share phenotypic similarities. Interestingly, ACBD3 associates with the Golgi apparatus in neurons and interphase progenitor cells but becomes cytosolic after Golgi fragmentation during mitosis, when Numb activity is needed to distinguish the two daughter cells. Accordingly, cytosolic ACBD3 can act synergistically with Numb to specify cell fates, and its continuing presence during the progenitor cell cycle inhibits neuron production. We propose that Golgi fragmentation and reconstitution during cell cycle differentially regulate Numb signaling through changes in ACBD3 subcellular distribution and represent a mechanism for coupling cell-fate specification and cell-cycle progression.

Drosophila protein kinase D is broadly expressed and a fraction localizes to the Golgi compartment

Protein kinase D belongs to the subfamily of CaMK. In mammals, three isoforms are known. They have been linked to diverse cellular functions including regulation of cell proliferation, differentiation, apoptosis and motility as well as secretory transport from the trans-Golgi compartment to the plasma membrane. Accordingly, the mammalian PKDs show different intracellular locations, with reported dynamic redistribution, between cytosol, Golgi, plasma membranes and the nucleus, depending on the cell type and exogenous stimuli. The genome of Drosophila melanogaster harbours just one, highly conserved PKD homologue, which is expressed throughout development. PKD mRNA expression during late embryogenesis is restricted to ectodermal derivatives including those involved in cuticle secretion. In imaginal tissues, transcription appears more uniform. PKD protein is detected predominantly in the cytosol with an enrichment in lateral apodemes of late embryos as well as in larval fascicles. In secretory tissues like salivary glands, the protein is concentrated in dotted structures. A PKD-GFP transgene reveals a similar punctuate protein accumulation juxtaposed to a resident Golgi-marker. In cultured cells, transfected Drosophila PKD-GFP colocalizes with a marker of the trans-Golgi compartment like human PKD1-GFP. Similar to the mammalian homologues, Drosophila PKD may be multifunctional including a role in secretory transport in accordance with its subcellular distribution.

centrosomin's beautiful sister (cbs) encodes a GRIP-domain protein that marks Golgi inheritance and functions in the centrosome cycle in Drosophila

The mechanism of inheritance of the Golgi complex is an important problem in cell biology. In this study, we examine the localization and function of a Golgi protein encoded by centrosomin's beautiful sister (cbs) during cleavage in Drosophila melanogaster. Cbs contains a GRIP domain that is 57% identical to vertebrate Golgin-97. Cbs undergoes a dramatic relocalization during mitosis from the cytoplasm to an association with chromosomes from late prometaphase to early telophase, by a transport mechanism that requires the GRIP domain and Arl1, the product of the Arf72A locus. Additionally, Cbs remains independent of the endoplasmic reticulum throughout cleavage. The use of RNAi, Arf72A mutant analysis and ectopic expression of the GRIP domain, shows that cycling of Cbs during mitosis is required for the centrosome cycle. The effects on the centrosome cycle depend on Cbs concentration and Cbs transport from the cytoplasm to DNA. When Cbs levels are reduced centrosomes fail to mature, and when Cbs transport is impeded by ectopic expression of the GRIP domain, centrosomes undergo hypertrophy. We propose that, Cbs is a trans-Golgi protein that links Golgi inheritance to the cell cycle and the Drosophila Golgi is more vertebrate-like than previously recognized.

TANGOing along the protein secretion pathway

An RNAi screen in Drosophila cells has identified about 100 TANGO proteins, which may regulate protein exocytosis or secretion.

Although the organization and functions of the constitutive secretory pathway have been intensively studied for decades, a recent genome-wide RNAi screen in Drosophila cells has identified about 100 genes encoding novel so-called TANGO proteins (for transport and Golgi organization) that may be direct regulators of various aspects of protein exocytosis or secretion.

The secretory membrane system in the Drosophila syncytial blastoderm embryo exists as functionally compartmentalized units around individual nuclei

Drosophila melanogaster embryogenesis begins with 13 nuclear division cycles within a syncytium. This produces >6,000 nuclei that, during the next division cycle, become encased in plasma membrane in the process known as cellularization. In this study, we investigate how the secretory membrane system becomes equally apportioned among the thousands of syncytial nuclei in preparation for cellularization. Upon nuclear arrival at the cortex, the endoplasmic reticulum (ER) and Golgi were found to segregate among nuclei, with each nucleus becoming surrounded by a single ER/Golgi membrane system separate from adjacent ones. The nuclear-associated units of ER and Golgi across the syncytial blastoderm produced secretory products that were delivered to the plasma membrane in a spatially restricted fashion across the embryo. This occurred in the absence of plasma membrane boundaries between nuclei and was dependent on centrosome-derived microtubules. The emergence of secretory membranes that compartmentalized around individual nuclei in the syncytial blastoderm is likely to ensure that secretory organelles are equivalently partitioned among nuclei at cellularization and could play an important role in the establishment of localized gene and protein expression patterns within the early embryo.

Functional genomics reveals genes involved in protein secretion and Golgi organization

Yeast genetics and in vitro biochemical analysis have identified numerous genes involved in protein secretion. As compared with yeast, however, the metazoan secretory pathway is more complex and many mechanisms that regulate organization of the Golgi apparatus remain poorly characterized. We performed a genome-wide RNA-mediated interference screen in a Drosophila cell line to identify genes required for constitutive protein secretion. We then classified the genes on the basis of the effect of their depletion on organization of the Golgi membranes. Here we show that depletion of class A genes redistributes Golgi membranes into the endoplasmic reticulum, depletion of class B genes leads to Golgi fragmentation, depletion of class C genes leads to aggregation of Golgi membranes, and depletion of class D genes causes no obvious change. Of the 20 new gene products characterized so far, several localize to the Golgi membranes and the endoplasmic reticulum.

Polarized Transport of Frizzled along the Planar Microtubule Arrays in Drosophila Wing Epithelium

Cells in a variety of developmental contexts sense extracellular cues that are given locally on their surfaces, and subsequently amplify the initial signal to achieve cell polarization. Drosophila wing cells acquire planar polarity along the proximal-distal (P-D) axis, in which the amplification of the presumptive cue involves assembly of a multiprotein complex that spans distal and proximal boundaries of adjacent cells. Here we pursue the mechanisms that place one of the components, Frizzled (Fz), at the distal side. Intracellular particles of GFP-tagged Fz moved preferentially toward distal boundaries before Fz::GFP and other components were tightly localized at the P/D cortex. Arrays of microtubules (MTs) were approximately oriented along the P-D axis and these MTs contributed to the formation of the cortical complex. Furthermore, there appeared to be a bias in the P-D MTs, with slightly more plus ends oriented distally. The hypothesis of polarized vesicular trafficking of Fz is discussed.

Drosophila S2 cells produce multiple forms of carboxypeptidase D with different intracellular distributions

Carboxypeptidase D (CPD) functions in the processing of proteins that transit the secretory pathway, and is present in all vertebrates examined as well as Drosophila. Several forms of CPD mRNA were previously found in Drosophila that resulted from differential splicing of the gene. In the present study, Northern blot, reverse transcriptase PCR, and Western blot analysis showed that each splice variant occurs in a single cell type, the Drosophila-derived Schneider 2 (S2) cell line. The short forms containing a single carboxypeptidase domain were secreted from the S2 cells while the long forms containing three carboxypeptidase domains, a transmembrane domain, and one of two different cytosolic tails were retained in the cell. To investigate the role of the two different C-terminal tail sequences (tail-1 and tail-2) that result from the differential splicing within exon 8, constructs containing a reporter protein (albumin) attached to the transmembrane domain and tail-1 or tail-2 of CPD were expressed in S2 cells and a mouse pituitary cell line (AtT20 cells). Immunofluorescence analysis revealed different intracellular distributions of the two constructs, with the tail-2 construct showing considerable overlap with a Golgi marker. The two C-terminal tail sequences also resulted in different internalization efficiencies from the cell surface in both cell lines. Interestingly, the distribution and routing of the tail-2 form of Drosophila CPD in the AtT20 cells are similar to the previously characterized endogenous mouse CPD protein, indicating that the elements for this trafficking have been conserved between Drosophila and mammals.

Distinct functional units of the Golgi complex in Drosophila cells

A striking variety of glycosylation occur in the Golgi complex in a protein-specific manner, but how this diversity and specificity are achieved remains unclear. Here we show that stacked fragments (units) of the Golgi complex dispersed in Drosophila imaginal disk cells are functionally diverse. The UDP-sugar transporter FRINGE-CONNECTION (FRC) is localized to a subset of the Golgi units distinct from those harboring SULFATELESS (SFL), which modifies glucosaminoglycans (GAGs), and from those harboring the protease RHOMBOID (RHO), which processes the glycoprotein SPITZ (SPI). Whereas the glycosylation and function of NOTCH are affected in imaginal disks of frc mutants, those of SPI and of GAG core proteins are not, even though FRC transports a broad range of glycosylation substrates, suggesting that Golgi units containing FRC and those containing SFL or RHO are functionally separable. Distinct Golgi units containing FRC and RHO in embryos could also be separated biochemically by immunoisolation techniques. We also show that Tn-antigen glycan is localized only in a subset of the Golgi units distributed basally in a polarized cell. We propose that the different localizations among distinct Golgi units of molecules involved in glycosylation underlie the diversity of glycan modification.

The exocyst component Sec5 is present on endocytic vesicles in the oocyte of Drosophila melanogaster

The exocyst is an octameric complex required for polarized secretion. Some components of the exocyst are found on the plasma membrane, whereas others are recruited to Golgi membranes, suggesting that exocyst assembly tethers vesicles to their site of fusion. We have found that in Drosophila melanogaster oocytes the majority of the exocyst component Sec5 is unexpectedly present in clathrin-coated pits and vesicles at the plasma membrane. In oocytes, the major substrate for clathrin-dependent endocytosis is the vitellogenin receptor Yolkless. A truncation mutant of Sec5 (sec5(E13)) allows the formation of normally sized oocytes but with greatly reduced yolk uptake. We find that in sec5(E13) oocytes Yolkless accumulates aberrantly in late endocytic compartments, indicating a defect in the endocytic cycling of the receptor. An analogous truncation of the yeast SEC5 gene results in normal secretion but a temperature-sensitive defect in endocytic recycling. Thus, the exocyst may act in both Golgi to plasma membrane traffic and endocytic cycling, and hence in oocytes is recruited to clathrin-coated pits to facilitate the rapid recycling of Yolkless.

Essential function of Drosophila Sec6 in apical exocytosis of epithelial photoreceptor cells

Polarized exocytosis plays a major role in development and cell differentiation but the mechanisms that target exocytosis to specific membrane domains in animal cells are still poorly understood. We characterized Drosophila Sec6, a component of the exocyst complex that is believed to tether secretory vesicles to specific plasma membrane sites. sec6 mutations cause cell lethality and disrupt plasma membrane growth. In developing photoreceptor cells (PRCs), Sec6 but not Sec5 or Sec8 shows accumulation at adherens junctions. In late PRCs, Sec6, Sec5, and Sec8 colocalize at the rhabdomere, the light sensing subdomain of the apical membrane. PRCs with reduced Sec6 function accumulate secretory vesicles and fail to transport proteins to the rhabdomere, but show normal localization of proteins to the apical stalk membrane and the basolateral membrane. Furthermore, we show that Rab11 forms a complex with Sec5 and that Sec5 interacts with Sec6 suggesting that the exocyst is a Rab11 effector that facilitates protein transport to the apical rhabdomere in Drosophila PRCs.

The Drosophila PAR-1 spacer domain is required for lateral membrane association and for polarization of follicular epithelial cells

The Ser/Thr kinases of the PAR-1/MARK/Kin1 family are conserved regulators of polarity in epithelial and non-epithelial cells . Drosophila PAR-1 localizes laterally in the follicular epithelium of the ovary , where it has been shown to function at two distinct levels: It stabilizes the cytoskeleton and it regulates apical-basal polarity by directly inhibiting lateral assembly of the apical aPKC/Bazooka/PAR-6 complex . However, it has been unclear how lateral localization of Drosophila PAR-1 is achieved and whether this localization contributes to epithelial polarity in vivo. Here we show that, through its spacer domain, Drosophila PAR-1 accumulates on the lateral plasma membrane (PM) in cells of the follicular epithelium (FE). Rescue experiments indicate that in FE cells PAR-1 kinase activity is essential for all the described functions of PAR-1. In contrast, the spacer domain of PAR-1 is required for apical-basal polarity and growth control but is dispensable for microtubule (MT) stabilization. Our data indicate that the spacer domain of PAR-1 is required for lateral PM localization of PAR-1 kinase and for development of a polarized FE.

Golgi structure in stress sensing and apoptosis

The Golgi complex in mammalian cells is composed of polarized stacks of flattened cisternal membranes. Stacks are connected by tubules forming a reticular network of membranes closely associated with the microtubule-organizing center. While the Golgi structure is important for the efficient processing of secretory cargo, the organization of the mammalian Golgi complex may indicate potential functions in addition to the processing and sorting of cargo. Similar to the endoplasmic reticulum stress response pathway, the Golgi complex may initiate signaling pathways to alleviate stress, and if irreparable, trigger apoptosis. Here, we review recent experimental evidence suggesting that the elaborate structure of the Golgi complex in mammalian cells may have evolved to sense and transduce stress signals.

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.