Cdc2 kinase-dependent disassembly of endoplasmic reticulum (ER) exit sites inhibits ER-to-Golgi vesicular transport during mitosis

Aurora kinase A-mediated phosphorylation triggers structural alteration of Rab1A to enhance ER complexity during mitosis

Morphological rearrangement of the endoplasmic reticulum (ER) is critical for metazoan mitosis. Yet, how the ER is remodeled by the mitotic signaling remains unclear. Here, we report that mitotic Aurora kinase A (AURKA) employs a small GTPase, Rab1A, to direct ER remodeling. During mitosis, AURKA phosphorylates Rab1A at Thr75. Structural analysis demonstrates that Thr75 phosphorylation renders Rab1A in a constantly active state by preventing interaction with GDP-dissociation inhibitor (GDI). Activated Rab1A is retained on the ER and induces the oligomerization of ER-shaping protein RTNs and REEPs, eventually triggering an increase of ER complexity. In various models, from Caenorhabditis elegans and Drosophila to mammals, inhibition of Rab1AThr75 phosphorylation by genetic modifications disrupts ER remodeling. Thus, our study reveals an evolutionarily conserved mechanism explaining how mitotic kinase controls ER remodeling and uncovers a critical function of Rab GTPases in metaphase.

Fine-tuning cell organelle dynamics during mitosis by small GTPases

During mitosis, the allocation of genetic material concurs with organelle transformation and distribution. The coordination of genetic material inheritance with organelle dynamics directs accurate mitotic progression, cell fate determination, and organismal homeostasis. Small GTPases belonging to the Ras superfamily regulate various cell organelles during division. Being the key regulators of membrane dynamics, the dysregulation of small GTPases is widely associated with cell organelle disruption in neoplastic and non-neoplastic diseases, such as cancer and Alzheimer's disease. Recent discoveries shed light on the molecular properties of small GTPases as sophisticated modulators of a remarkably complex and perfect adaptors for rapid structure reformation. This review collects current knowledge on small GTPases in the regulation of cell organelles during mitosis and highlights the mediator role of small GTPase in transducing cell cycle signaling to organelle dynamics during mitosis.

Characteristics and Functions of the Yip1 Domain Family (YIPF), Multi-Span Transmembrane Proteins Mainly Localized to the Golgi Apparatus

Yip1 domain family (YIPF) proteins are multi-span, transmembrane proteins mainly localized in the Golgi apparatus. YIPF proteins have been found in virtually all eukaryotes, suggesting that they have essential function(s). Saccharomyces cerevisiae contains four YIPFs: Yip1p, Yif1p, Yip4p, and Yip5p. Early analyses in S. cerevisiae indicated that Yip1p and Yif1p bind to each other and play a role in budding of transport vesicles and/or fusion of vesicles to target membranes. However, the molecular basis of their functions remains unclear. Analysis of YIPF proteins in mammalian cells has yielded significant clues about the function of these proteins. Human cells have nine family members that appear to have overlapping functions. These YIPF proteins are divided into two sub-families: YIPFα/Yip1p and YIPFβ/Yif1p. A YIPFα molecule forms a complex with a specific partner YIPFβ molecule. In the most broadly hypothesized scenario, a basic tetramer complex is formed from two molecules of each partner YIPF protein, and this tetramer forms a higher order oligomer. Three distinct YIPF protein complexes are formed from pairs of YIPFα and YIPFβ proteins. These are differently localized in either the early, middle, or late compartments of the Golgi apparatus and are recycled between adjacent compartments. Because a YIPF protein is predicted to have five transmembrane segments, a YIPF tetramer complex is predicted to have 20 transmembrane segments. This high number of transmembrane segments suggests that YIPF complexes function as channels, transporters, or transmembrane receptors. Here, the evidence from functional studies of YIPF proteins obtained during the last two decades is summarized and discussed.

Establishment and phenotyping of disease model cells created by cell-resealing technique

Cell-based assays are growing in importance for screening drugs and investigating their mechanisms of action. Most of the assays use so-called “normal” cell strain because it is difficult to produce cell lines in which the disease conditions are reproduced. In this study, we used a cell-resealing technique, which reversibly permeabilizes the plasma membrane, to develop diabetic (Db) model hepatocytes into which cytosol from diabetic mouse liver had been introduced. Db model hepatocytes showed several disease-specific phenotypes, namely disturbance of insulin-induced repression of gluconeogenic gene expression and glucose secretion. Quantitative image analysis and principal component analysis revealed that the ratio of phosphorylated Akt (pAkt) to Akt was the best index to describe the difference between wild-type and Db model hepatocytes. By performing image-based drug screening, we found pioglitazone, a PPARγ agonist, increased the pAkt/Akt ratio, which in turn ameliorated the insulin-induced transcriptional repression of the gluconeogenic gene phosphoenolpyruvate carboxykinase 1. The disease-specific model cells coupled with image-based quantitative analysis should be useful for drug development, enabling the reconstitution of disease conditions at the cellular level and the discovery of disease-specific markers.

The G1 Cyclin-dependent Kinase CRK1 in Trypanosoma brucei Regulates Anterograde Protein Transport by Phosphorylating the COPII Subunit Sec31

Transport of secretory proteins from the endoplasmic reticulum to the Golgi is mediated by the coat protein II (COPII) complex comprising a Sec23-Sec24 heterodimer and a Sec13-Sec31 heterotetramer. The mechanisms underlying COPII-mediated protein trafficking have been well defined, but the extent of regulation of this secretory machinery by cellular signaling pathways remains poorly understood. Here, we report that CRK1, a G1 cyclin-dependent kinase in Trypanosoma brucei, regulates anterograde protein trafficking by phosphorylating Sec31. Depletion of CRK1 abolished anterograde transport of the secretory protein and disrupted the localization of multiple Golgi proteins, reminiscent of Sec31 depletion. CRK1 phosphorylates Sec31 at multiple serine/threonine sites, and mutation of these phosphosites to alanine recapitulates the protein trafficking defects caused by Sec31 depletion. Mutation of these CRK1 phosphosites to aspartate restored Sec31 function. Taken together, these results uncover a novel function of CRK1 in anterograde protein trafficking and elucidate the mechanistic role of CRK1 in protein trafficking through regulation of the COPII subunit Sec31.

Quantitative phosphoproteomic analyses of the inferior parietal lobule from three different pathological stages of Alzheimer's disease

Alzheimer's disease (AD), the most common age-related neurodegenerative disorder, is clinically characterized by progressive neuronal loss resulting in loss of memory and dementia. AD is histopathologically characterized by the extensive distribution of senile plaques and neurofibrillary tangles, and synapse loss. Amnestic mild cognitive impairment (MCI) is generally accepted to be an early stage of AD. MCI subjects have pathology and symptoms that fall on the scale intermediately between 'normal' cognition with little or no pathology and AD. A rare number of individuals, who exhibit normal cognition on psychometric tests but whose brains show widespread postmortem AD pathology, are classified as 'asymptomatic' or 'preclinical' AD (PCAD). In this study, we evaluated changes in protein phosphorylation states in the inferior parietal lobule of subjects with AD, MCI, PCAD, and control brain using a 2-D PAGE proteomics approach in conjunction with Pro-Q Diamond phosphoprotein staining. Statistically significant changes in phosphorylation levels were found in 19 proteins involved in energy metabolism, neuronal plasticity, signal transduction, and oxidative stress response. Changes in the disease state phosphoproteome may provide insights into underlying mechanisms for the preservation of memory with expansive AD pathology in PCAD and the progressive memory loss in amnestic MCI that escalates to the dementia and the characteristic pathology of AD brain.

Mechanisms and Regulation of the Mitotic Inheritance of the Golgi Complex

In mammalian cells, the Golgi complex is structured in the form of a continuous membranous system composed of stacks connected by tubular bridges: the "Golgi ribbon." At the onset of mitosis, the Golgi complex undergoes a multi-step fragmentation process that is required for its correct partition into the dividing cells. Importantly, inhibition of Golgi disassembly results in cell-cycle arrest at the G2 stage, which indicates that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Moreover, mitotic Golgi disassembly correlates with the release of a set of Golgi-localized proteins that acquire specific functions during mitosis, such as mitotic spindle formation and regulation of the spindle checkpoint. Most of these events are regulated by small GTPases of the Arf and Rab families. Here, we review recent studies that are revealing the fundamental mechanisms, the molecular players, and the biological significance of mitotic inheritance of the Golgi complex in mammalian cells. We also briefly comment on how Golgi partitioning is coordinated with mitotic progression.

On the move: organelle dynamics during mitosis

A cell constitutes the minimal self-replicating unit of all organisms, programmed to propagate its genome as it proceeds through mitotic cell division. The molecular processes entrusted with ensuring high fidelity of DNA replication and subsequent segregation of chromosomes between daughter cells have therefore been studied extensively. However, to process the information encoded in its genome a cell must also pass on its non-genomic identity to future generations. To achieve productive sharing of intracellular organelles, cells have evolved complex mechanisms of organelle inheritance. Many membranous compartments undergo vast spatiotemporal rearrangements throughout mitosis. These controlled organizational changes are crucial to enabling completion of the division cycle and ensuring successful progeny. Herein we review current understanding of intracellular organelle segregation during mitotic division in mammalian cells, with a focus on compartment organization and integrity throughout the inheritance process.

Positive feedback between Golgi membranes, microtubules and ER exit sites directs de novo biogenesis of the Golgi

The Golgi complex is the central organelle of the secretory pathway. It undergoes dynamic changes during the cell cycle, but how it acquires and maintains its complex structure is unclear. To address this question, we have used laser nanosurgery to deplete BSC1 cells of the Golgi complex and have monitored its biogenesis by quantitative time-lapse microscopy and correlative electron microscopy. After Golgi depletion, endoplasmic reticulum (ER) export is inhibited and the number of ER exit sites (ERES) is reduced and does not increase for several hours. Occasional fusion of small post-ER carriers to form the first larger structures triggers a rapid and drastic growth of Golgi precursors, due to the capacity of these structures to attract more carriers by microtubule nucleation and to stimulate ERES biogenesis. Increasing the chances of post-ER carrier fusion close to ERES by depolymerizing microtubules results in the acceleration of Golgi and ERES biogenesis. Taken together, on the basis of our results, we propose a self-organizing principle of the early secretory pathway that integrates Golgi biogenesis, ERES biogenesis and the organization of the microtubule network by positive-feedback loops.

VAMP4 is required to maintain the ribbon structure of the Golgi apparatus

The Golgi apparatus forms a twisted ribbon-like network in the juxtanuclear region of vertebrate cells. Vesicle-associated membrane protein 4 (VAMP4), a v-SNARE protein expressed exclusively in the vertebrate trans-Golgi network (TGN), plays a role in retrograde trafficking from the early endosome to the TGN, although its precise function within the Golgi apparatus remains unclear. To determine whether VAMP4 plays a functional role in maintaining the structure of the Golgi apparatus, we depleted VAMP4 gene expression using RNA interference technology. Depletion of VAMP4 from HeLa cells led to fragmentation of the Golgi ribbon. These fragments were not uniformly distributed throughout the cytoplasm, but remained in the juxtanuclear area. Electron microscopy and immunohistochemistry showed that in the absence of VAMP4, the length of the Golgi stack was shortened, but Golgi stacking was normal. Anterograde trafficking was not impaired in VAMP4-depleted cells, which contained intact microtubule arrays. Depletion of the cognate SNARE partners of VAMP4, syntaxin 6, syntaxin 16, and Vti1a also disrupted the Golgi ribbon structure. Our findings suggested that the maintenance of Golgi ribbon structure requires normal retrograde trafficking from the early endosome to the TGN, which is likely to be mediated by the formation of VAMP4-containing SNARE complexes.

Lipid phosphate phosphatase 3 participates in transport carrier formation and protein trafficking in the early secretory pathway

The inhibition of phosphatidic acid phosphatase (PAP) activity by propanolol indicates that diacylglycerol (DAG) is required for the formation of transport carriers at the Golgi and for retrograde trafficking to the ER. Here we report that the PAP2 family member lipid phosphate phosphatase 3 (LPP3, also known as PAP2b) localizes in compartments of the secretory pathway from ER export sites to the Golgi complex. The depletion of human LPP3: (i) reduces the number of tubules generated from the ER-Golgi intermediate compartment and the Golgi, with those formed from the Golgi being longer in LPP3-silenced cells than in control cells; (ii) impairs the Rab6-dependent retrograde transport of Shiga toxin subunit B from the Golgi to the ER, but not the anterograde transport of VSV-G or ssDsRed; and (iii) induces a high accumulation of Golgi-associated membrane buds. LPP3 depletion also reduces levels of de novo synthesized DAG and the Golgi-associated DAG contents. Remarkably, overexpression of a catalytically inactive form of LPP3 mimics the effects of LPP3 knockdown on Rab6-dependent retrograde transport. We conclude that LPP3 participates in the formation of retrograde transport carriers at the ER-Golgi interface, where it transitorily cycles, and during its route to the plasma membrane.

Multi-step down-regulation of the secretory pathway in mitosis: a fresh perspective on protein trafficking

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non-dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down-regulated during mitosis. Moreover, the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin-dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport.

Is unconventional secretion inhibited during cell division by Cdk1 activity?

A process of unconventional secretion that is dependent on the Golgi stacking protein GRASP and multiple components of the autophagy machinery has recently been documented for several cytoplasmic and membrane protein. Classical secretion via the exocytic pathway is inhibited during cell division in animal cells, as key membrane compartments, particularly the Golgi, are disassembled and fragmented. The question as to whether unconventional secretion is likewise inhibited during mitosis has not been explored. This mode of secretion supposedly bypasses the Golgi. However, GRASP and Vps34 (a key autophagy protein) are both substrates of the cell cycle regulating cyclin-dependent kinase 1 (Cdk1), and their activities are apparently inhibited by Cdk1 phosphorylation. Is unconventional secretion therefore similarly inhibited during cell division like conventional secretion? The story may yet turn out to be more complicated.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

A resealed-cell system for analyzing pathogenic intracellular events: perturbation of endocytic pathways under diabetic conditions

Cell-based assay systems that can serve as cellular models of aberrant function in pathogenic organs would be novel and useful tools for screening drugs and clarifying the molecular mechanisms of various diseases. We constructed model cells that replicated the conditions in diabetic hepatocytes by using the cell resealing technique, which enables the exchange of cytosol. The plasma membrane of HeLa cells was permeabilized with the streptococcal toxin streptolysin O, and cytosol that had been prepared from wild-type or db/db diabetic mice was introduced into the resulting semi-intact cells. By resealing the plasma membrane by exposure to Ca²⁺, we created WT or Db model cells, in which the cytosolic conditions replicated those of healthy or diabetic liver. Interestingly, phosphorylation of p38 MAPK was promoted, whereas the level of endosomal phosphatidylinositol-3-phosphate was decreased, in Db cells. We investigated several endocytic pathways in WT and Db cells, and found that retrograde endosome-to-Golgi transport was delayed in a p38 MAPK-dependent manner in Db cells. Furthermore, the degradation pathway of the EGF receptor from endosomes to lysosomes was enhanced in Db cells, and this did not depend on the activation of p38 MAPK. The disease model cell system should become a powerful tool for the detection of aberrant processes in cells under pathogenic conditions and for therapeutic applications.

BBF2H7-mediated Sec23A pathway is required for endoplasmic reticulum-to-Golgi trafficking in dermal fibroblasts to promote collagen synthesis

Collagen fibers, structural elements responsible for mechanical strength in skin, are synthesized constitutively in response to cytokines such as IGF-I. However, little is known about their intracellular trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus during synthesis. We demonstrate herein that the BBF2 human homolog on chromosome 7 (BBF2H7)-mediated Sec23A pathway is involved in regulation of intracellular procollagen trafficking. The mRNA and protein expression of BBF2H7, Sec23A, and type I and III collagen (COL1 and COL3) was induced by IGF-I stimulation. In addition, the cleaved form of BBF2H7 was detected in IGF-I-treated cultures, indicating that activation occurs concurrently with its expression. Knockdown with small interfering RNAs targeting BBF2H7 caused a significant reduction in the expression of COL1 and COL3, regardless of IGF-I treatment. Both mitogen-activated protein kinase and phosphatidylinositol-3 kinase pathways via IGF-I receptor activation were required for BBF2H7 induction. Using immunofluorescence microscopy, we showed that Golgi apparatus dysmorphology is due to coat protein complex II vehicle hypoplasia caused by the absence of BBF2H7 and Sec23A. The BBF2H7-mediated Sec23A pathway was required for ER-to-Golgi procollagen trafficking to promote collagen synthesis. This role of growth factors such as IGF-I, which to our knowledge is previously unreported, suggests antiaging strategies.

A rapid and quantitative coat protein complex II vesicle formation assay using luciferase reporters

The majority of protein export from the endoplasmic reticulum (ER) is facilitated by coat protein complex II (COPII). The COPII proteins deform the ER membrane into vesicles at the ER exit sites. During the vesicle formation step, the COPII proteins load cargo molecules into the vesicles. Formation of COPII vesicles has been reconstituted in vitro in yeast and in mammalian systems. These in vitro COPII vesicle formation assays involve incubation of microsomal membranes and purified COPII proteins with nucleotides. COPII vesicles are separated from the microsomes by differential centrifugation. Interestingly, the efficiency of the COPII vesicle formation with purified recombinant mammalian COPII proteins is lower than that with cytosol, suggesting that an additional cytosolic factor(s) is involved in this process. Indeed, other studies have also implicated additional factors. To facilitate biochemical identification of such regulators, a rapid and quantitative COPII vesicle formation assay is necessary because the current assay is lengthy. To expedite this assay, we generated luciferase reporter constructs. The reporter proteins were packaged into COPII vesicles and yielded quantifiable luminescent signals, resulting in a rapid and quantitative COPII vesicle formation assay.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

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.

Hydrogen peroxide depletes phosphatidylinositol-3-phosphate from endosomes in a p38 MAPK-dependent manner and perturbs endocytosis

Phosphatidylinositol-3-phosphate (PI3P) is a lipid that is enriched specifically in early endosomes. Given that early endosomes containing PI3P act as a microdomain to recruit proteins that contain a PI3P-binding domain (FYVE domain), the equilibrium between the production and degradation of PI3P influences a variety of processes, including endocytosis and signal transduction via endosomes. In the study reported herein, we have developed a novel analytical method to quantify the amount of PI3P in endosomes by introducing a GST-2xFYVE protein probe into semi-intact cells. The GST-2xFYVE probe was targeted specifically to intracellular PI3P-containing endosomes, which retained their small punctate structure, and allowed the semi-quantitative measurement of intracellular PI3P. Using the method, we found that treatment of HeLa cells with H(2)O(2) decreased the amount of PI3P in endosomes in a p38 MAPK-dependent manner. In addition, H(2)O(2) treatment delayed transport through various endocytic pathways, especially post-early endosome transport; the retrograde transport of cholera toxin was especially dependent on the amount of PI3P in endosomes. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Golgi-associated GSK3beta regulates the sorting process of post-Golgi membrane trafficking

Glycogen synthase kinase β (GSK3β) phosphorylates many substrates in mammalian cells, and functions in many physiological processes. We observed that GSK3β knockdown by siRNA perturbed both Golgi morphology in HeLa cells and the anterograde transport of cation-independent mannose 6-phosphate receptor (CI-M6PR) from the trans-Golgi network (TGN) to prelysosomal compartments (PLC), diverting it to the exocytic pathway. Moreover, we demonstrate that a portion of GSK3β was localized to the TGN through the Golgi peripheral protein p230 and that this localization regulated CLASP2 phosphorylation. Our results also show that GSK3β knockdown resulted in accumulation of CLASP2 at microtubule plus ends at the cell periphery. Our findings support the hypothesis that GSK3β at the TGN acts as a guide, activates exocytic transport, and redirects CI-M6PR from transport to the PLC into the exocytic pathway by regulating the affinity of CLASPs for microtubules.

Structural and functional implications of phosphorylation and acetylation in the regulation of the AAA+ protein p97

p97, also known as VCP (valosin-containing protein), is a hexameric AAA+ ATPase that participates in a variety of cellular processes. It is believed that p97 mediates these processes through the binding of various adaptor proteins. Many factors govern adaptor binding and the regulatory mechanisms are not yet well understood. Sites of phosphorylation and acetylation on p97 have been identified and such post-translational modifications may be involved in regulating p97 function. Phosphorylation and, to a lesser extent, acetylation of p97 have been shown to modify its properties - for example, by modulating adaptor binding and directing subcellular localization. These modifications have been implicated in a number of p97-mediated processes, including misfolded protein degradation, membrane fusion, and transcription factor activation. This review describes the known phosphorylation and acetylation sites on p97 and discusses their possible structural and functional implications.

Mitotic division of the mammalian Golgi apparatus

Successful cell reproduction requires faithful duplication and proper segregation of cellular contents, including not only the genome but also intracellular organelles. Since the Golgi apparatus is an essential organelle of the secretory pathway, its accurate inheritance is therefore of importance to sustain cellular function. Regulation of Golgi division and its coordination with cell cycle progression involves a series of sequential events that are subjected to a precise spatiotemporal control. Here, we summarize the current knowledge about the underlying mechanisms, the molecular players and the biological relevance of this process, particularly in mammalian cells, and discuss the unsolved problems and future perspectives opened by the recent studies.

Yip1A regulates the COPI-independent retrograde transport from the Golgi complex to the ER

Yip1A, a mammalian homologue of yeast Yip1p, is a multi-spanning membrane protein that is considered to be involved in transport between the endoplasmic reticulum (ER) and the Golgi. However, the precise role of Yip1A in mammalian cells remains unclear. We show here that endogenous Yip1A is localized to the ER-Golgi intermediate compartment (ERGIC). Knockdown of Yip1A by RNAi did not induce morphological changes in the Golgi, ER, or ERGIC. By analyzing a number of intracellular transport pathways, we found that Yip1A knockdown delayed the transport of Shiga toxin from the Golgi to the ER, but did not affect the anterograde transport of VSVGts045. We also found that a recombinant protein that corresponded to the N-terminal domain of Yip1A inhibited the COPI-independent retrograde transport of GFP-tagged galactosyltransferase, GT-GFP, but not the COPI-dependent retrograde transport of p58/ERGIC53. Furthermore, we found that Yip1A knockdown resulted in the dissociation of Rab6 from the membranes. These results suggested that Yip1A has a role in COPI-independent retrograde transport from the Golgi to the ER and regulates the membrane recruitment of Rab6.

VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death

Frontotemporal lobar degeneration (FTLD) with inclusion body myopathy and Paget disease of bone is a rare, autosomal dominant disorder caused by mutations in the VCP (valosin-containing protein) gene. The disease is characterized neuropathologically by frontal and temporal lobar atrophy, neuron loss and gliosis, and ubiquitin-positive inclusions (FTLD-U), which are distinct from those seen in other sporadic and familial FTLD-U entities. The major component of the ubiquitinated inclusions of FTLD with VCP mutation is TDP-43 (TAR DNA-binding protein of 43 kDa). TDP-43 proteinopathy links sporadic amyotrophic lateral sclerosis, sporadic FTLD-U, and most familial forms of FTLD-U. Understanding the relationship between individual gene defects and pathologic TDP-43 will facilitate the characterization of the mechanisms leading to neurodegeneration. Using cell culture models, we have investigated the role of mutant VCP in intracellular trafficking, proteasomal function, and cell death and demonstrate that mutations in the VCP gene 1) alter localization of TDP-43 between the nucleus and cytosol, 2) decrease proteasome activity, 3) induce endoplasmic reticulum stress, 4) increase markers of apoptosis, and 5) impair cell viability. These results suggest that VCP mutation-induced neurodegeneration is mediated by several mechanisms.

CDC2/SPDY transiently associates with endoplasmic reticulum exit sites during oocyte maturation

BACKGROUND

Mammalian oocytes acquire competence to be fertilized during meiotic maturation. The protein kinase CDC2 plays a pivotal role in several key maturation events, in part through controlled changes in CDC2 localization. Although CDC2 is involved in initiation of maturation, a detailed analysis of CDC2 localization at the onset of maturation is lacking. In this study, the subcellular distribution of CDC2 and its regulatory proteins cyclin B and SPDY in combination with several organelle markers at the onset of pig oocyte maturation has been investigated.

RESULTS

Our results demonstrate that CDC2 transiently associates with a single domain, identified as a cluster of endoplasmic reticulum (ER) exit sites (ERES) by the presence of SEC23, in the cortex of maturing porcine oocytes prior to germinal vesicle break down. Inhibition of meiosis resumption by forskolin treatment prevented translocation of CDC2 to this ERES cluster. Phosphorylated GM130 (P-GM130), which is a marker for fragmented Golgi, localized to ERES in almost all immature oocytes and was not affected by forskolin treatment. After removal of forskolin from the culture media, the transient translocation of CDC2 to ERES was accompanied by a transient dispersion of P-GM130 into the ER suggesting a role for CDC2 in redistributing Golgi components that have collapsed into ERES further into the ER during meiosis. Finally, we show that SPDY, rather than cyclin B, colocalizes with CDC2 at ERES, suggesting a role for the CDC2/SPDY complex in regulating the secretory pathway during oocyte maturation.

CONCLUSION

Our data demonstrate the presence of a novel structure in the cortex of porcine oocytes that comprises ERES and transiently accumulates CDC2 prior to germinal vesicle breakdown. In addition, we show that SPDY, but not cyclin B, localizes to this ERES cluster together with CDC2.

PKC zeta-mediated phosphorylation controls budding of the pre-chylomicron transport vesicle

Dietary triacylglycerols are absorbed by enterocytes and packaged in the endoplasmic reticulum (ER) in the intestinal specific lipoprotein, the chylomicron, for export into mesenteric lymph. Chylomicrons exit the ER in an ER-to-Golgi transport vesicle, the pre-chylomicron transport vesicle (PCTV), which is the rate-limiting step in the transit of chylomicrons across the cell. Here, we focus on potential mechanisms of control of the PCTV-budding step from the intestinal ER. We incubated intestinal ER with intestinal cytosol and ATP to cause PCTV budding. The budding reaction was inhibited by 60 nM of the PKC inhibitor Gö 6983, suggesting the importance of PKCzeta in the generation of PCTV. Immunodepletion of PKCzeta from the cytosol and the use of washed ER greatly inhibited the generation of PCTVs, but was restored following the addition of recombinant PKCzeta. Intestinal ER incubated with intestinal cytosol and [gamma-(32)P]ATP under conditions supporting the generation of PCTVs showed the phosphorylation of a 9-kDa band following autoradiography. The phosphorylation of this protein correlated with the generation of PCTVs but not the formation of protein vesicles and was inhibited by depletion of PKCzeta. Phosphorylation of the 9-kDa protein was restored following the addition of recombinant PKCzeta. The association of the 9-kDa protein with proteins that are important for PCTV budding was phosphorylation dependent. We conclude that PKCzeta activity is required for PCTV budding from intestinal ER, and is associated with phosphorylation of a 9-kDa protein that might regulate PCTV budding.

Myt1 protein kinase is essential for Golgi and ER assembly during mitotic exit

Myt1 was originally identified as an inhibitory kinase for Cdc2 (Cdk1), the master engine of mitosis, and has been thought to function, together with Wee1, as a negative regulator of mitotic entry. In this study, we report an unexpected finding that Myt1 is essential for Golgi and endoplasmic reticulum (ER) assembly during telophase in mammalian cells. Our analyses reveal that both cyclin B1 and cyclin B2 serve as targets of Myt1 for proper Golgi and ER assembly to occur. Thus, our results show that Myt1-mediated suppression of Cdc2 activity is not indispensable for the regulation of a broad range of mitotic events but is specifically required for the control of intracellular membrane dynamics during mitosis.

p38 MAPK regulates COPII recruitment

Here, we investigate regulation of coat protein complex II (COPII) recruitment onto ER export sites in permeabilized cells. In cytosols from nocodazole treated HeLa cells we find COPII loading is inhibited. The stress kinase p38 MAPK is activated in these cytosols and COPII loading can be rescued by depletion of p38 MAPK alpha or by the p38 MAPK inhibitor (SB203580) but not by inhibition/depletion of cdc2. These observations indicate regulation of the early secretory pathway by p38 MAPK.

Mitosis controls the Golgi and the Golgi controls mitosis

In mammals, the Golgi complex is structured in the form of a continuous membranous system composed of up to 100 stacks connected by tubular bridges, the 'Golgi ribbon'. During mitosis, the Golgi undergoes extensive fragmentation through a multistage process that allows its correct partitioning and inheritance by daughter cells. Strikingly, this Golgi fragmentation is required not only for inheritance but also for mitotic entrance itself, since its block results in the arrest of the cell cycle in G2. This is called the 'Golgi mitotic checkpoint'. Recent studies have identified the severing of the ribbon into its constituent stacks during early G2 as the precise stage of Golgi fragmentation that controls mitotic entry. This opens new ways to elucidate the mechanism of the Golgi checkpoint.

A CDK-related kinase regulates the length and assembly of flagella in Chlamydomonas

Little is known about how cells regulate the size of their organelles. In this study, we find that proper flagellar length control in Chlamydomonas reinhardtii requires the activity of a new member of the cyclin-dependent kinase (CDK) family, which is encoded by the LF2 (long flagella 2) gene. This novel CDK contains all of the important residues that are essential for kinase activity but lacks the cyclin-binding motif PSTAIRE. Analysis of genetic lesions in a series of lf2 mutant alleles and site-directed mutagenesis of LF2p reveals that improper flagellar length and defective flagellar assembly correlate with the extent of disruption of conserved kinase structures or residues by mutations. LF2p appears to interact with both LF1p and LF3p in the cytoplasm, as indicated by immunofluorescence localization, sucrose density gradients, cell fractionation, and yeast two-hybrid experiments. We propose that LF2p is the catalytic subunit of a regulatory kinase complex that controls flagellar length and flagellar assembly.

p97 adaptor choice regulates organelle biogenesis

The AAA protein p97 requires adaptor-like cofactors for its numerous cellular functions. In this issue of Developmental Cell, Uchiyama et al. (2006) identify p37 as a p97 adaptor that is required constitutively for ER and Golgi membrane fusion, analogous to the mitotic membrane fusion role of the adaptor p47. Their study suggests that related p97 adaptors involved in similar cellular pathways can be subject to differential regulation.

Exit from mitosis triggers Chs2p transport from the endoplasmic reticulum to mother-daughter neck via the secretory pathway in budding yeast

Budding yeast chitin synthase 2 (Chs2p), which lays down the primary septum, localizes to the mother–daughter neck in telophase. However, the mechanism underlying the timely neck localization of Chs2p is not known. Recently, it was found that a component of the exocyst complex, Sec3p–green fluorescent protein, arrives at the neck upon mitotic exit. It is not clear whether the neck localization of Chs2p, which is a cargo of the exocyst complex, was similarly regulated by mitotic exit. We report that Chs2p was restrained in the endoplasmic reticulum (ER) during metaphase. Furthermore, mitotic exit was sufficient to cause Chs2p neck localization specifically by triggering the Sec12p-dependent transport of Chs2p out of the ER. Chs2p was “forced” prematurely to the neck by mitotic kinase inactivation at metaphase, with chitin deposition occurring between mother and daughter cells. The dependence of Chs2p exit from the ER followed by its transport to the neck upon mitotic exit ensures that septum formation occurs only after the completion of mitotic events.

Consequence of gastrin-releasing peptide receptor activation in a human colon cancer cell line: a proteomic approach

Gastrin-releasing peptide (GRP) and its receptor (GRPR) are aberrantly up-regulated in colon cancer. When expressed, they act as morphogens, retaining tumor cells in a better differentiated state and retarding metastasis. To identify targets activated in response to GRPR signaling we studied Caco-2 and HT-29 cells, colon cancer cell lines that expresses GRPR as a function of confluence. Total cell protein was extracted from pre-confluent cells (expressing GRP/GRPR) cultured in serum-free media in the presence or absence of GRPR-specific antagonist; as well as from confluent cells that do not express GRPR. Overall, we identified 5 proteins that are specifically down-regulated after GRP/GRPR expression: Bach2, creatine kinase B, p47, and two that could not be identified; and 6 proteins that are up-regulated: gephyrin, HSP70, HP1, ICAM-1, ACAT, and one that could not be identified. These findings suggest that the mechanism(s) by which GRP/GRPR mediate its morphogenic effects in colon cancer involve the actions of a number of hitherto unappreciated proteins.

NSF/SNAPs and p97/p47/VCIP135 are sequentially required for cell cycle-dependent reformation of the ER network

The endoplasmic reticulum (ER) has a characteristic polygonal structure with hallmark three-way junctions. In a previous paper, we reconstituted the disruption of the pre-existing ER network using mitotic cytosol from HeLa cells in streptolysin O (SLO)-permeabilized CHO-HSP cells (stably expressing GFP-HSP47). In addition, we found that interphase cytosol induced reformation of the disrupted ER network into a continuous network structure. Here, we show that the reformation of the ER network is accomplished through two sequential fusion reactions. The first process is mediated by NSF/alpha and gamma-SNAPs, and involves the generation of typical membranous intermediate structures that connect the disrupted ER tubules. A subsequent fusion is mediated by p97/p47/VCIP135, which has been shown to be required for homotypic fusion events in Golgi cisternae regrowth after mitosis. In addition, we also found that both fusion processes involve the t-SNARE, syntaxin 18.

The maintenance of the endoplasmic reticulum network is regulated by p47, a cofactor of p97, through phosphorylation by cdc2 kinase

The endoplasmic reticulum (ER) has a characteristic complex polygonal structure with hallmark three-way junctions in many types of cells. To investigate the mechanisms responsible for maintaining the ER network, we established ER disassembly and reassembly assays in semi-intact Chinese hamster ovary (CHO) cells that constitutively expressed heat shock protein-47 fused to the green fluorescent protein (GFP-HSP47) as an ER marker (the cells are referred to as CHO-HSP cells). Using these assays, we found that maintenance of the ER network required cytosol and adenosine triphosphate/guanosine 5'-triphosphate (ATP/GTP) hydrolysis, but not actin filaments or microtubules. We also showed that the ER network was disrupted upon addition of either N-ethylmaleimide-treated cytosol after washing semi-intact cells with high salt solution or mitotic cytosol in nocodazole-treated semi-intact CHO-HSP cells. The disrupted ER network induced by mitotic cytosol was reformed by the addition of interphase cytosol. In addition, we found that p47, a cofactor of p97, was essential for the maintenance of the ER network, and that phosphorylation of p47 by cdc2 kinase resulted in ER network disruption by mitotic cytosol. Taken together, these results imply that the maintenance of the ER network requires a membrane fusion process mediated by p97/p47, and that cell cycle-dependent morphological changes of the ER network are regulated through phosphorylation/dephosphorylation of p47.

PCTAIRE protein kinases interact directly with the COPII complex and modulate secretory cargo transport

The export of secretory cargo from the endoplasmic reticulum is mediated by the COPII complex. In common with other aspects of intracellular transport, this step is regulated by protein kinase signalling. Recruitment of the COPII complex to the membrane is known to require ATP and to be blocked by the protein kinase inhibitor H-89. The identity of the specific protein kinase or kinases involved remains equivocal. Here we show that the Sec23p subunit of COPII interacts with PCTAIRE protein kinases. This interaction is shown using two-hybrid screening, direct binding and immunoprecipitation. Inhibition of PCTAIRE kinase activity by expression of a kinase-dead mutant, or specific depletion of PCTAIRE using RNAi, leads to defects in early secretory pathway function including cargo transport, as well as vesicular-tubular transport carrier (VTC) and Golgi localization. These data show a role for PCTAIRE protein kinase function in membrane traffic through the early secretory pathway.

Nm23H2 facilitates coat protein complex II assembly and endoplasmic reticulum export in mammalian cells

The cytosolic coat protein complex II (COPII) mediates vesicle formation from the endoplasmic reticulum (ER) and is essential for ER-to-Golgi trafficking. The minimal machinery for COPII assembly is well established. However, additional factors may regulate the process in mammalian cells. Here, a morphological COPII assembly assay using purified COPII proteins and digitonin-permeabilized cells has been applied to demonstrate a role for a novel component of the COPII assembly pathway. The factor was purified and identified by mass spectrometry as Nm23H2, one of eight isoforms of nucleoside diphosphate kinase in mammalian cells. Importantly, recombinant Nm23H2, as well as a catalytically inactive version, promoted COPII assembly in vitro, suggesting a noncatalytic role for Nm23H2. Consistent with a function for Nm23H2 in ER export, Nm23H2 localized to a reticular network that also stained for the ER marker calnexin. Finally, an in vivo role for Nm23H2 in COPII assembly was confirmed by isoform-specific knockdown of Nm23H2 by using short interfering RNA. Knockdown of Nm23H2, but not its most closely related isoform Nm23H1, resulted in diminished COPII assembly at steady state and reduced kinetics of ER export. These results strongly suggest a previously unappreciated role for Nm23H2 in mammalian ER export.