Dynamin-mediated Internalization of Caveolae

The dynamins comprise an expanding family of ubiquitously expressed 100-kD GTPases that have been implicated in severing clathrin-coated pits during receptor-mediated endocytosis. Currently, it is unclear whether the different dynamin isoforms perform redundant functions or participate in distinct endocytic processes. To define the function of dynamin II in mammalian epithelial cells, we have generated and characterized peptide-specific antibodies to domains that either are unique to this isoform or conserved within the dynamin family. When microinjected into cultured hepatocytes these affinity-purified antibodies inhibited clathrin-mediated endocytosis and induced the formation of long plasmalemmal invaginations with attached clathrin-coated pits. In addition, clusters of distinct, nonclathrin-coated, flask-shaped invaginations resembling caveolae accumulated at the plasma membrane of antibody-injected cells. In support of this, caveola-mediated endocytosis of labeled cholera toxin B was inhibited in antibody-injected hepatocytes. Using immunoisolation techniques an anti-dynamin antibody isolated caveolar membranes directly from a hepatocyte postnuclear membrane fraction. Finally, double label immunofluorescence microscopy revealed a striking colocalization between dynamin and the caveolar coat protein caveolin. Thus, functional in vivo studies as well as ultrastructural and biochemical analyses indicate that dynamin mediates both clathrin-dependent endocytosis and the internalization of caveolae in mammalian cells.     

Association of a dynamin-like protein with the Golgi apparatus in mammalian cells

Dynamins are a family of 100-kD GTPases comprised of at least three distinct gene products and multiple alternatively spliced variants. Homologies with the shibire gene product in Drosophila melanogaster and with Vps1p and Dnm1p in Saccharomyces cerevisiae suggest that dynamins play an important role in vesicular transport. Morphological studies have localized brain dynamin to coated pits and tubular invaginations at the plasma membrane, where it is believed to facilitate the formation of endocytic vesicles. Because similar membrane-budding events occur at the Golgi apparatus and multiple dynamin isoforms exist, we have studied the distribution of dynamins in mammalian cells. To this end, we generated and characterized peptide-specific antibodies directed against conserved regions of the dynamin family. By immunoblot analysis, these antibodies reacted specifically with a 100-kD protein in fibroblasts that sedimented with membranes and microtubules in vitro in a manner similar to brain dynamin. By immunofluorescence microscopy, these antibodies strongly labeled the Golgi complex in cultured fibroblasts and melanocytes, as confirmed by double labeling with a Golgi-specific antibody. Furthermore, Western blot analysis showed significant enrichment of a 100-kD dynamin band in Golgi fractions isolated from the liver. To substantiate these findings, we use a specific antidynamin antibody to immunoisolate Golgi membranes from subcellular Golgi fractions, as determined by EM and immunoblot analysis. This study provides the first morphological and biochemical evidence that a dynamin-like protein associates with the Golgi apparatus in mammalian cells, and suggests that dynamin-related proteins may have multiple cytoplasmic distributions. The potential contributions of dynamin to the secretory and endocytic pathways are discussed.     

A feedback loop between dynamin and actin recruitment during clathrin-mediated endocytosis

Clathrin-mediated endocytosis proceeds by a sequential series of reactions catalyzed by discrete sets of protein machinery. The final reaction in clathrin-mediated endocytosis is membrane scission, which is mediated by the large guanosine triophosphate hydrolase (GTPase) dynamin and which may involve the actin-dependent recruitment of N-terminal containing BIN/Amphiphysin/RVS domain containing (N-BAR) proteins. Optical microscopy has revealed a detailed picture of when and where particular protein types are recruited in the ～20-30 s preceding scission. Nevertheless, the regulatory mechanisms and functions that underpin protein recruitment are not well understood. Here we used an optical assay to investigate the coordination and interdependencies between the recruitment of dynamin, the actin cytoskeleton, and N-BAR proteins to individual clathrin-mediated endocytic scission events. These measurements revealed that a feedback loop exists between dynamin and actin at sites of membrane scission. The kinetics of dynamin, actin, and N-BAR protein recruitment were modulated by dynamin GTPase activity. Conversely, acute ablation of actin dynamics using latrunculin-B led to a ～50% decrease in the incidence of scission, an ～50% decrease in the amplitude of dynamin recruitment, and abolished actin and N-BAR recruitment to scission events. Collectively these data suggest that dynamin, actin, and N-BAR proteins work cooperatively to efficiently catalyze membrane scission. Dynamin controls its own recruitment to scission events by modulating the kinetics of actin and N-BAR recruitment to sites of scission. Conversely actin serves as a dynamic scaffold that concentrates dynamin and N-BAR proteins at sites of scission.     

Cortactin is a component of clathrin-coated pits and participates in receptor-mediated endocytosis

The actin cytoskeleton is believed to contribute to the formation of clathrin-coated pits, although the specific components that connect actin filaments with the endocytic machinery are unclear. Cortactin is an F-actin-associated protein, localizes within membrane ruffles in cultured cells, and is a direct binding partner of the large GTPase dynamin. This direct interaction with a component of the endocytic machinery suggests that cortactin may participate in one or several endocytic processes. Therefore, the goal of this study was to test whether cortactin associates with clathrin-coated pits and participates in receptor-mediated endocytosis. Morphological experiments with either anti-cortactin antibodies or expressed red fluorescence protein-tagged cortactin revealed a striking colocalization of cortactin and clathrin puncta at the ventral plasma membrane. Consistent with these observations, cells microinjected with these antibodies exhibited a marked decrease in the uptake of labeled transferrin and low-density lipoprotein while internalization of the fluid marker dextran was unchanged. Cells expressing the cortactin Src homology three domain also exhibited markedly reduced endocytosis. These findings suggest that cortactin is an important component of the receptor-mediated endocytic machinery, where, together with actin and dynamin, it regulates the scission of clathrin pits from the plasma membrane. Thus, cortactin provides a direct link between the dynamic actin cytoskeleton and the membrane pinchase dynamin that supports vesicle formation during receptor-mediated endocytosis.     

Phosphorylation Regulates the Endocytic Function of the Yeast Dynamin-Related Protein Vps1

The family of dynamin proteins is known to function in many eukaryotic membrane fusion and fission events. The yeast dynamin-related protein Vps1 functions at several stages of membrane trafficking, including Golgi apparatus to endosome and vacuole, peroxisomal fission, and endocytic scission. We have previously shown that in its endocytic role, Vps1 functions with the amphiphysin heterodimer Rvs161/Rvs167 to facilitate scission and release of vesicles. Phosphoproteome studies of Saccharomyces cerevisiae have identified a phosphorylation site in Vps1 at serine 599. In this study, we confirmed this phosphorylation event, and we reveal that, like Rvs167, Vps1 can be phosphorylated by the yeast cyclin-associated kinase Pho85 in vivo and in vitro. The importance of this posttranslational modification was revealed when mutagenesis of S599 to a phosphomimetic or nonphosphorylatable form caused defects in endocytosis but not in other functions associated with Vps1. Mutation to nonphosphorylatable valine inhibited the Rvs167 interaction, while both S599V and S599D caused defects in vesicle scission, as shown by both live-cell imaging and electron microscopy of endocytic invaginations. Our data support a model in which phosphorylation and dephosphorylation of Vps1 promote distinct interactions and highlight the importance of such regulatory events in facilitating sequential progression of the endocytic process.     

Constitutive activated Cdc42-associated kinase (Ack) phosphorylation at arrested endocytic clathrin-coated pits of cells that lack dynamin

Clathrin-mediated endocytosis is a fundamental cellular process conserved from yeast to mammals and is an important endocytic route for the internalization of many specific cargos, including activated growth factor receptors. Here we examined changes in tyrosine phosphorylation, a representative output of growth factor receptor signaling, in cells in which endocytic clathrin-coated pits are frozen at a deeply invaginated state, that is, cells that lack dynamin (fibroblasts from dynamin 1, dynamin 2 double conditional knockout mice). The major change observed in these cells relative to wild-type cells was an increase in the phosphorylation state, and thus activation, of activated Cdc42-associated kinase (Ack), a nonreceptor tyrosine kinase. Ack is concentrated at clathrin-coated pits, and binds clathrin heavy chain via two clathrin boxes. RNA interference-based approaches and pharmacological manipulations further demonstrated that the phosphorylation of Ack requires both clathrin assembly into endocytic clathrin-coated pits and active Cdc42. These findings reveal a link between progression of clathrin-coated pits to endocytic vesicles and an activation-deactivation cycle of Ack.     

A Highlights from MBoC Selection: A dimeric equilibrium intermediate nucleates Drp1 reassembly on mitochondrial membranes for fission

The GTPase dynamin-related protein 1 (Drp1) catalyzes mitochondrial division, but the mechanisms remain poorly understood. Much of what is attributed to Drp1’s mechanism of action in mitochondrial membrane fission parallels that of prototypical dynamin in endocytic vesicle scission. Unlike the case for dynamin, however, no lipid target for Drp1 activation at the mitochondria has been identified. In addition, the oligomerization properties of Drp1 have not been well established. We show that the mitochondria-specific lipid cardiolipin is a potent stimulator of Drp1 GTPase activity, as well as of membrane tubulation. We establish further that under physiological conditions, Drp1 coexists as two morphologically distinct polymeric species, one nucleotide bound in solution and the other membrane associated, which equilibrate via a dimeric assembly intermediate. With two mutations, C300A and C505A, that shift Drp1 polymerization equilibria in opposite directions, we demonstrate that dimers, and not multimers, potentiate the reassembly and reorganization of Drp1 for mitochondrial membrane remodeling both in vitro and in vivo.     

A Highlights from MBoC Selection: Dynamin recruitment and membrane scission at the neck of a clathrin-coated pit

Dynamin, the GTPase required for clathrin-mediated endocytosis, is recruited to clathrin-coated pits in two sequential phases. The first is associated with coated pit maturation; the second, with fission of the membrane neck of a coated pit. Using gene-edited cells that express dynamin2-EGFP instead of dynamin2 and live-cell TIRF imaging with single-molecule EGFP sensitivity and high temporal resolution, we detected the arrival of dynamin at coated pits and defined dynamin dimers as the preferred assembly unit. We also used live-cell spinning-disk confocal microscopy calibrated by single-molecule EGFP detection to determine the number of dynamins recruited to the coated pits. A large fraction of budding coated pits recruit between 26 and 40 dynamins (between 1 and 1.5 helical turns of a dynamin collar) during the recruitment phase associated with neck fission; 26 are enough for coated vesicle release in cells partially depleted of dynamin by RNA interference. We discuss how these results restrict models for the mechanism of dynamin-mediated membrane scission.     

Dominant-negative inhibition of receptor-mediated endocytosis by a dynamin-1 mutant with a defective pleckstrin homology domain

The dynamins are 100 kDa GTPases involved in the scission of endocytic vesicles from the plasma membrane [1]. Dynamin-1 is present in solution as a tetramer [2], and undergoes further self-assembly following its recruitment to coated pits to form higher-order oligomers that resemble 'collars' around the necks of nascent coated buds [1] [3]. GTP hydrolysis by dynamin in these collars is thought to accompany the 'pinching off' of endocytic vesicles [1] [4]. Dynamin contains a pleckstrin homology (PH) domain that binds phosphoinositides [5] [6], which in turn enhance both the GTPase activity [5] [7] [8] and self-assembly [9] [10] of dynamin. We recently showed that the dynamin PH domain binds phosphoinositides only when it is oligomeric [6]. Here, we demonstrate that interactions between the dynamin PH domain and phosphoinositides are important for dynamin function in vivo. Full-length dynamin-1 containing mutations that abolish phosphoinositide binding by its PH domain was a dominant-negative inhibitor of receptor-mediated endocytosis. Mutated dynamin-1 with both a defective PH domain and impaired GTP binding and hydrolysis also inhibited receptor-mediated endocytosis. These findings suggest that the role of the PH domain in dynamin function differs from that seen for other PH domains. We propose that high-avidity binding to phosphoinositide-rich regions of the membrane by the multiple PH domains in a dynamin oligomer is critical for dynamin's ability to complete vesicle budding.     

The dynamin family of mechanoenzymes: pinching in new places

The large GTPase dynamin is a mechanoenzyme that mediates the liberation of nascent clathrin-coated pits from the plasma membrane during endocytosis. Recently, this enzyme has been demonstrated to comprise an extensive family of related proteins that have been implicated in a large variety of vesicle trafficking events during endocytosis, secretion and even maintenance of mitochondrial form. The potential contributions by the dynamin family to these diverse but related functions are discussed.     

Imaging single endocytic events reveals diversity in clathrin, dynamin and vesicle dynamics

The dynamics of clathrin-mediated endocytosis can be assayed using fluorescently tagged proteins and total internal reflection fluorescence microscopy. Many of these proteins, including clathrin and dynamin, are soluble and changes in fluorescence intensity can be attributed either to membrane/vesicle movement or to changes in the numbers of individual molecules. It is important for assays to discriminate between physical membrane events and the dynamics of molecules. Two physical events in endocytosis were investigated: vesicle scission from the plasma membrane and vesicle internalization. Single vesicle analysis allowed the characterization of dynamin and clathrin dynamics relative to scission and internalization. We show that vesicles remain proximal to the plasma membrane for variable amounts of time following scission, and that uncoating of clathrin can occur before or after vesicle internalization. The dynamics of dynamin also vary with respect to scission. Results from assays based on physical events suggest that disappearance of fluorescence from the evanescent field should be re-evaluated as an assay for endocytosis. These results illustrate the heterogeneity of behaviors of endocytic vesicles and the importance of establishing suitable evaluation criteria for biophysical processes.     

Membrane Tethering and Nucleotide-dependent Conformational Changes Drive Mitochondrial Genome Maintenance (Mgm1) Protein-mediated Membrane Fusion

Cellular membrane remodeling events such as mitochondrial dynamics, vesicle budding, and cell division rely on the large GTPases of the dynamin superfamily. Dynamins have long been characterized as fission molecules; however, how they mediate membrane fusion is largely unknown. Here we have characterized by cryo-electron microscopy and in vitro liposome fusion assays how the mitochondrial dynamin Mgm1 may mediate membrane fusion. Using cryo-EM, we first demonstrate that the Mgm1 complex is able to tether opposing membranes to a gap of ∼15 nm, the size of mitochondrial cristae folds. We further show that the Mgm1 oligomer undergoes a dramatic GTP-dependent conformational change suggesting that s-Mgm1 interactions could overcome repelling forces at fusion sites and that ultrastructural changes could promote the fusion of opposing membranes. Together our findings provide mechanistic details of the two known in vivo functions of Mgm1, membrane fusion and cristae maintenance, and more generally shed light onto how dynamins may function as fusion proteins.     

An InCytes from MBC Selection: Membrane Insertion of the Pleckstrin Homology Domain Variable Loop 1 Is Critical for Dynamin-catalyzed Vesicle Scission

The GTPase dynamin catalyzes the scission of deeply invaginated clathrin-coated pits at the plasma membrane, but the mechanisms governing dynamin-mediated membrane fission remain poorly understood. Through mutagenesis, we have altered the hydrophobic nature of the membrane-inserting variable loop 1 (VL1) of the pleckstrin homology (PH) domain of dynamin-1 and demonstrate that its stable insertion into the lipid bilayer is critical for high membrane curvature generation and subsequent membrane fission. Dynamin PH domain mutants defective in curvature generation regain function when assayed on precurved membrane templates in vitro, but they remain defective in the scission of clathrin-coated pits in vivo. These results demonstrate that, in concert with dynamin self-assembly, PH domain membrane insertion is essential for fission and vesicle release in vitro and for clathrin-mediated endocytosis in vivo.     

Dynamin‐Actin Cross Talk Contributes to Phagosome Formation and Closure

Phagocytosis is a mechanism used by macrophages to internalize and eliminate microorganisms or cellular debris. It relies on profound rearrangements of the actin cytoskeleton that is the driving force allowing plasma membrane extension around the particle. The closure step of phagocytosis, however, remains poorly defined. We used a dedicated experimental setup with Total Internal Reflection Fluorescence Microscopy (TIRFM) to monitor phagosome formation and closure in three dimensions in living cells. We show that dynamin‐2, which mediates the scission of endocytic vesicles, was recruited early and concomitantly with actin during phagosome formation. Dynamin‐2 accumulated at the site of phagosome closure in living macrophages. Inhibition of its activity with dominant negative mutants or drugs demonstrated that dynamin‐2 is implicated in actin dynamics and pseudopod extension. Depolymerization of actin led to impaired dynamin‐2 recruitment or activity. Finally, we show that dynamin‐2 plays a critical role in the effective scission of the phagosome from the plasma membrane. Thus, we establish that a cross talk between actin and dynamin takes place for phagosome formation and closure before dynamin functions for scission.      

Dynasore, a cell-permeable inhibitor of dynamin

Dynamin is essential for clathrin-dependent coated vesicle formation. It is required for membrane budding at a late stage during the transition from a fully formed pit to a pinched-off vesicle. Dynamin may also fulfill other roles during earlier stages of vesicle formation. We have screened about 16,000 small molecules and have identified 1, named here dynasore, that interferes in vitro with the GTPase activity of dynamin1, dynamin2, and Drp1, the mitochondrial dynamin, but not of other small GTPases. Dynasore acts as a potent inhibitor of endocytic pathways known to depend on dynamin by rapidly blocking coated vesicle formation within seconds of dynasore addition. Two types of coated pit intermediates accumulate during dynasore treatment, U-shaped, half formed pits and O-shaped, fully formed pits, captured while pinching off. Thus, dynamin acts at two steps during clathrin coat formation; GTP hydrolysis is probably needed at both steps.     

Role for dynamin in late endosome dynamics and trafficking of the cation-independent mannose 6-phosphate receptor

It is well established that dynamin is involved in clathrin-dependent endocytosis, but relatively little is known about possible intracellular functions of this GTPase. Using confocal imaging, we found that endogenous dynamin was associated with the plasma membrane, the trans-Golgi network, and a perinuclear cluster of cation-independent mannose 6-phosphate receptor (CI-MPR)-containing structures. By electron microscopy (EM), it was shown that these structures were late endosomes and that the endogenous dynamin was preferentially localized to tubulo-vesicular appendices on these late endosomes. Upon induction of the dominant-negative dynK44A mutant, confocal microscopy demonstrated a redistribution of the CI-MPR in mutant-expressing cells. Quantitative EM analysis of the ratio of CI-MPR to lysosome-associated membrane protein-1 in endosome profiles revealed a higher colocalization of the two markers in dynK44A-expressing cells than in control cells. Western blot analysis showed that dynK44A-expressing cells had an increased cellular procathepsin D content. Finally, EM revealed that in dynK44A-expressing cells, endosomal tubules containing CI-MPR were formed. These results are in contrast to recent reports that dynamin-2 is exclusively associated with endocytic structures at the plasma membrane. They suggest instead that endogenous dynamin also plays an important role in the molecular machinery behind the recycling of the CI-MPR from endosomes to the trans-Golgi network, and we propose that dynamin is required for the final scission of vesicles budding from endosome tubules.     

The role of dynamin and its binding partners in coated pit invagination and scission

Plasma membrane clathrin-coated vesicles form after the directed assembly of clathrin and the adaptor complex, AP2, from the cytosol onto the membrane. In addition to these structural components, several other proteins have been implicated in clathrin-coated vesicle formation. These include the large molecular weight GTPase, dynamin, and several Src homology 3 (SH3) domain-containing proteins which bind to dynamin via interactions with its COOH-terminal proline/arginine-rich domain (PRD). To understand the mechanism of coated vesicle formation, it is essential to determine the hierarchy by which individual components are targeted to and act in coated pit assembly, invagination, and scission.To address the role of dynamin and its binding partners in the early stages of endocytosis, we have used well-established in vitro assays for the late stages of coated pit invagination and coated vesicle scission. Dynamin has previously been shown to have a role in scission of coated vesicles. We show that dynamin is also required for the late stages of invagination of clathrin-coated pits. Furthermore, dynamin must bind and hydrolyze GTP for its role in sequestering ligand into deeply invaginated coated pits.We also demonstrate that the SH3 domain of endophilin, which binds both synaptojanin and dynamin, inhibits both late stages of invagination and also scission in vitro. This inhibition results from a reduction in phosphoinositide 4,5-bisphosphate levels which causes dissociation of AP2, clathrin, and dynamin from the plasma membrane. The dramatic effects of the SH3 domain of endophilin led us to propose a model for the temporal order of addition of endophilin and its binding partner synaptojanin in the coated vesicle cycle.     

Localization of serum resistance‐associated protein in Trypanosoma brucei rhodesiense and transgenic Trypanosoma brucei brucei

African trypanosomes infect a broad range of mammals, but humans and some higher primates are protected by serum trypanosome lytic factors that contain apolipoprotein L1 (ApoL1). In the human‐infective subspecies of Trypanosoma brucei, Trypanosoma brucei rhodesiense, a gene product derived from the variant surface glycoprotein gene family member, serum resistance‐associated protein (SRA protein), protects against ApoL1‐mediated lysis. Protection against trypanosome lytic factor requires the direct interaction between SRA protein and ApoL1 within the endocytic apparatus of the trypanosome, but some uncertainty remains as to the precise mechanism and location of this interaction. In order to provide more insight into the mechanism of SRA‐mediated resistance to trypanosome lytic factor, we assessed the localization of SRA in T. b. rhodesiense EATRO3 using a novel monoclonal antibody raised against SRA together with a set of well‐characterized endosomal markers. By three‐dimensional deconvolved immunofluorescence single‐cell analysis, combined with double‐labelling immunoelectron microscopy, we found that ≈ 50% of SRA protein localized to the lysosome, with the remaining population being distributed through the endocytic pathway, but apparently absent from the flagellar pocket membrane. These data suggest that the SRA/trypanolytic factor interaction is intracellular, with the concentration within the endosomes potentially crucial for ensuring a high efficiency.     

Characterization and developmentally regulated localization of the mitochondrial carrier protein homologue MCP6 from Trypanosoma brucei

Proteins of the mitochondrial carrier family (MCF) are located mainly in the inner mitochondrial membrane and mediate the transport of a large range of metabolic intermediates. The genome of Trypanosoma brucei harbors 29 genes encoding different MCF proteins. We describe here the characterization of MCP6, a novel T. brucei MCF protein. Sequence comparison and phylogenetic reconstruction revealed that MCP6 is closely related to different mitochondrial ADP/ATP and calcium-dependent solute carriers, including the ATP-Mg/Pi carrier of Homo sapiens. However, MCP6 lacks essential amino acids and sequence motifs conserved in these metabolite transporters, and functional reconstitution and transport assays with E. coli suggested that this protein indeed does not function as an ADP/ATP or ATP-Mg/Pi carrier. The subcellular localization of MCP6 is developmentally regulated: in bloodstream-form trypanosomes, the protein is predominantly glycosomal, whereas in the procyclic form, it is found mainly in the mitochondria. Depletion of MCP6 in procyclic trypanosomes resulted in growth inhibition, an increased cell size, aberrant numbers of nuclei and kinetoplasts, and abnormal kinetoplast morphology, suggesting that depletion of MCP6 inhibits division of the kinetoplast.     

Induction of mutant dynamin specifically blocks endocytic coated vesicle formation

Dynamin is the mammalian homologue to the Drosophila shibire gene product. Mutations in this 100-kD GTPase cause a pleiotropic defect in endocytosis. To further investigate its role, we generated stable HeLa cell lines expressing either wild-type dynamin or a mutant defective in GTP binding and hydrolysis driven by a tightly controlled, tetracycline- inducible promoter. Overexpression of wild-type dynamin had no effect. In contrast, coated pits failed to become constricted and coated vesicles failed to bud in cells overexpressing mutant dynamin so that endocytosis via both transferrin (Tfn) and EGF receptors was potently inhibited. Coated pit assembly, invagination, and the recruitment of receptors into coated pits were unaffected. Other vesicular transport pathways, including Tfn receptor recycling, Tfn receptor biosynthesis, and cathepsin D transport to lysosomes via Golgi-derived coated vesicles, were unaffected. Bulk fluid-phase uptake also continued at the same initial rates as wild type. EM immunolocalization showed that membrane-bound dynamin was specifically associated with clathrin-coated pits on the plasma membrane. Dynamin was also associated with isolated coated vesicles, suggesting that it plays a role in vesicle budding. Like the Drosophila shibire mutant, HeLa cells overexpressing mutant dynamin accumulated long tubules, many of which remained connected to the plasma membrane. We conclude that dynamin is specifically required for endocytic coated vesicle formation, and that its GTP binding and hydrolysis activities are required to form constricted coated pits and, subsequently, for coated vesicle budding.