Evolution of chloroplast vesicle transport

Vesicle traffic plays a central role in eukaryotic transport. The presence of a vesicle transport system inside chloroplasts of spermatophytes raises the question of its phylogenetic origin. To elucidate the evolution of this transport system we analyzed organisms belonging to different lineages that arose from the first photosynthetic eukaryote, i.e. glaucocystophytes, chlorophytes, rhodophytes, and charophytes/embryophytes. Intriguingly, vesicle transport is not apparent in any group other than embryophytes. The transfer of this eukaryotic-type vesicle transport system from the cytosol into the chloroplast thus seems a late evolutionary development that was acquired by land plants in order to adapt to new environmental challenges.     

Retrograde vesicle transport in the Golgi

The Golgi apparatus is the central sorting and biosynthesis hub of the secretory pathway, and uses vesicle transport for the recycling of its resident enzymes. This system must operate with high fidelity and efficiency for the correct modification of secretory glycoconjugates. In this review, we discuss recent advances on how coats, tethers, Rabs and SNAREs cooperate at the Golgi to achieve vesicle transport. We cover the well understood vesicle formation process orchestrated by the COPI coat, and the comprehensively documented fusion process governed by a set of Golgi localised SNAREs. Much less clear are the steps in-between formation and fusion of vesicles, and we therefore provide a much needed update of the latest findings about vesicle tethering. The interplay between Rab GTPases, golgin family coiled-coil tethers and the conserved oligomeric Golgi (COG) complex at the Golgi are thoroughly evaluated.     

A vesicle transport system inside chloroplasts

Intracellular transport via membrane vesicle traffic is a well known feature of eukaryotic cells. Yet, no vesicle transport system has been described for prokaryotes or organelles of prokaryotic origin, such as chloroplasts and mitochondria. Here we show that chloroplasts possess a vesicle transport system with features similar to vesicle traffic in homotypic membrane fusion. Vesicle formation and fusion is affected by specific inhibitors, e.g. nucleotide analogues, protein phosphatase inhibitors and Ca2+ antagonists. This vesicle transfer is an ongoing process in mature chloroplasts indicating that it represents an important new pathway in the formation and maintenance of the thylakoid membranes.     

Connecting vesicle transport to the cytoskeleton

The cytoskeleton is crucial for the efficient and polarized transport of vesicles in intracellular membrane-sorting pathways. Recent studies have identified specific kinesin, dynein, and myosin motor proteins that mediate defined membrane transport steps. Important clues have also been uncovered about the nature of motor-protein receptors on vesicular cargoes and the molecular mechanisms of motor-protein regulation.     

An update on transport vesicle tethering

Abstract

Membrane trafficking involves the collection of cargo into nascent transport vesicles that bud off from a donor compartment, translocate along cytoskeletal tracks, and then dock and fuse with their target membranes. Docking and fusion involve initial interaction at a distance (tethering), followed by a closer interaction that leads to pairing of vesicle SNARE proteins (v-SNAREs) with target membrane SNAREs (t-SNAREs), thereby catalyzing vesicle fusion. When tethering cannot take place, transport vesicles accumulate in the cytoplasm. Tethering is generally carried out by two broad classes of molecules: extended, coiled-coil proteins such as the so-called Golgin proteins, or multi-subunit complexes such as the Exocyst, COG or Dsl complexes. This review will focus on the most recent advances in terms of our understanding of the mechanism by which tethers carry out their roles, and new structural insights into tethering complex transactions.     

An update on transport vesicle tethering

Membrane trafficking involves the collection of cargo into nascent transport vesicles that bud off from a donor compartment, translocate along cytoskeletal tracks, and then dock and fuse with their target membranes. Docking and fusion involve initial interaction at a distance (tethering), followed by a closer interaction that leads to pairing of vesicle SNARE proteins (v-SNAREs) with target membrane SNAREs (t-SNAREs), thereby catalyzing vesicle fusion. When tethering cannot take place, transport vesicles accumulate in the cytoplasm. Tethering is generally carried out by two broad classes of molecules: extended, coiled-coil proteins such as the so-called Golgin proteins, or multi-subunit complexes such as the Exocyst, COG or Dsl complexes. This review will focus on the most recent advances in terms of our understanding of the mechanism by which tethers carry out their roles, and new structural insights into tethering complex transactions.     

Two activators of microtubule-based vesicle transport

Cytoplasmic dynein purified by nucleotide dependent microtubule affinity has significant minus end-directed vesicle motor activity that decreases with each further purification step. Highly purified dynein causes membrane vesicles to bind but not move on microtubules. We exploited these observations to develop an assay for factors that, in combination with dynein, would permit minus end-directed vesicle motility. At each step of the purification, non-dynein fractions were recombined with dynein and assayed for vesicle motility. Two activating fractions were identified by this method. One, called Activator I, copurified with 20S dynein by velocity sedimentation but could be separated from it by ion exchange chromatography. Activator I increased only the frequency of dynein-driven vesicle movements. Activator II, sedimenting at 9S, increased both the frequency and velocity of vesicle transport and also supported plus end movements. Our results suggest that dynein-based motility is controlled at multiple levels and provide a preliminary characterization of two regulatory factors.     

Protein complexes in transport vesicle targeting

The transport of material between membrane-bounded organelles in eukaryotic cells requires the accurate delivery of different classes of carrier vesicles to specific target compartments. Recent studies indicate that different targeting reactions involve distinct protein complexes that act to mark the target organelle for incoming vesicles. This review focuses on the proteins and protein complexes that have been implicated in various targeting reactions.     

Microtubule-associated organelle and vesicle transport in fibroblasts

Allen Video-enhanced contrast/differential interference contrast (AVEC-DIC) microscopy was used in conjunction with video intensification immunofluorescence microscopy to demonstrate that organelles and vesicle (particles) can move in either direction along microtubular linear elements in fibroblasts [Hayden et al., 1983]. Since it is not possible to determine the number of microtubules making up a linear element with light microscopy alone, AVEC-DIC microscopy was used in conjunction with whole-mount electron microscopy to show bidirectional transport along a single microtubule [Hayden and Allen, 1984]. These studies demonstrate that the structural polarity of the microtubule does not determine the direction of particle motion, and since dynein is an asymetric molecule, a simple microtubule-dynein-particle hypothesis cannot explain bidirectional transport along a single microtubule. Very little is known about regulation of particle transport in most cell types. Human embryonic lung fibroblasts grown on glass coverslips were serum-deprived for 24 hours and re-fed with serumless medium; the particle translocations/5 minutes were then determined. The cells were then re-fed with either serumless medium, serum-containing medium, or serumless medium containing some bioactive factor, and the particle translocations/5 minutes were again determined for the same cells. Medium containing 10% fetal bovine serum inhibited particle translocation by 51.8%. Of the bioactive factors tested, only vasopressin produced a significant reduction in particle translocations (38%). This suggests that protein kinase C or calcium/calmodulin kinase could be involved in regulating particle transport.     

Function of SM protein in vesicle transport

Most cells contain various transport vesicles that target to different destinations. The underlying molecular mechanisms are highly conserved in evolution. Sec1/Munc-18 (SM) proteins play an important role on regulating vesicle transport by interacting with soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) at each vesicle fusion sites. SM proteins interact with syntaxin, an important component in SNARE complex, to regulate the assembly of SNARE complex, and promote overall membrane fusion process together with SNARE complex. This review summaries new research progresses of structure and function of SM protein.     

Dynein-mediated vesicle transport controls intracellular Salmonella replication

Salmonella typhimurium survives and replicates intracellular in a membrane-bound compartment, the Salmonella-containing vacuole (SCV). In HeLa cells, the SCV matures through interactions with the endocytic pathway, but Salmonella avoids fusion with mature lysosomes. The exact mechanism of the inhibition of phagolysosomal fusion is not understood. Rab GTPases control several proteins involved in membrane fusion and vesicular transport. The small GTPase Rab7 regulates the transport of and fusion between late endosomes and lysosomes and associates with the SCV. We show that the Rab7 GTPase cycle is not affected on the SCV. We then manipulated a pathway downstream of the small GTPase Rab7 in HeLa cells infected with Salmonella. Expression of the Rab7 effector RILP induces recruitment of the dynein/dynactin motor complex to the SCV. Subsequently, SCV fuse with lysosomes. As a result, the intracellular replication of Salmonella is inhibited. Activation of dynein-mediated vesicle transport can thus control intracellular survival of Salmonella.     

An actin-dependent mechanism for long-range vesicle transport

Intracellular transport is vital for the function, survival and architecture of every eukaryotic cell. Long-range transport in animal cells is thought to depend exclusively on microtubule tracks. This study reveals an unexpected actin-dependent but microtubule-independent mechanism for long-range transport of vesicles. Vesicles organize their own actin tracks by recruiting the actin nucleation factors Spire1, Spire2 and Formin-2, which assemble an extensive actin network from the vesicles' surfaces. The network connects the vesicles with one another and with the plasma membrane. Vesicles move directionally along these connections in a myosin-Vb-dependent manner to converge and to reach the cell surface. The overall outward-directed movement of the vesicle-actin network is driven by recruitment of vesicles to the plasma membrane in the periphery of the oocyte. Being organized in a dynamic vesicle-actin network allows vesicles to move in a local random manner and a global directed manner at the same time: they can reach any position in the cytoplasm, but also move directionally to the cell surface as a collective. Thus, collective movement within a network is a powerful and flexible mode of vesicle transport.     

The molecular control of transport vesicle fusion

The fusion of transport vesicles with the appropriate target membrane in constitutive transport is a complex and well-controlled process. Many of the molecular details of the reactions that result in this control are being revealed through the use of cell-free assays of protein transport as well as by the study of the molecular genetics of secretion in yeast. Kinetic analyses have indicated that several structural intermediates are formed after transport vesicles attach to their destination, but before they fuse with the appropriate membrane. Proteins that mediate the formation and processing of these intermediates have been identified. Included among these are small molecular weight GTP-binding proteins. This intricate set of reactions may ensure the fidelity of transport and guard the integrity of the organelles along the transport pathway.     

Toward the systems biology of vesicle transport

Systems biology aims to study complex biological processes, such as intracellular traffic, as a whole. Systematic genome-wide assays have the potential to identify the transport machinery, delineate pathways and uncover the molecular components of physiological processes that influence trafficking. A goal of this approach is to create predictive models of intracellular trafficking pathways that reflect these relationships. In this review, we highlight current genome-wide technologies of particular relevance to vesicle transport and describe recent applications of these technologies in the framework of systems biology. Systems approaches hold great promise for placing trafficking pathways in their cellular contexts.     

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.     

SM蛋白在膜泡运输中的功能

大多数细胞内都包含靶向不同细胞器的各种运输囊泡,其运输机制在进化上是高度保守的。Sec1/Munc-18(SM)蛋白在膜泡运输中起着重要的调控作用,它能够与SNARE(Soluble N-ethylmaleimide-sensitive factorattachment protein receptor)蛋白结合,共同在细胞内各个膜融合发生部位发挥重要作用。SM蛋白和SNARE复合体中的Syntaxin蛋白结合,调节SNARE复合体的装配,并与SNARE协同作用促进整个膜融合过程。文章对SM蛋白在结构和功能分析方面的最新研究进展进行了概述。     

Rab proteins, connecting transport and vesicle fusion

Small GTPases of the Rab family control timing of vesicle fusion. Fusion of two vesicles can only occur when they have been brought into close contact. Transport by microtubule- or actin-based motor proteins will facilitate this process in vivo. Ideally, transport and vesicle fusion are linked activities. Active, GTP-bound Rab proteins dock on specific compartments and are therefore perfect candidates to control transport of the different compartments. Recently, a number of Rab proteins were identified that control motor protein recruitment to their specific target membranes. By cycling through inactive and active states, Rab proteins are able to control motor protein-mediated transport and subsequent fusion of intracellular structures in both spatial and timed manners.     

Closing the GAP between polarity and vesicle transport

How are tight junctions maintained? In this issue of Cell, Wells et al. (2006) provide intriguing evidence for a new pathway that links polarity proteins and vesicle transport to the maintenance of tight junctions, through the control of Cdc42 by Rich1, a GTPase-activating protein.