Structural basis for endosomal targeting by the Bro1 domain

The ESCRT pathway and HIV-1 budding

HIV-1 Gag engages components of the ESCRT (endosomal sorting complex required for transport) pathway via so-called L (late-assembly) domains to promote virus budding. Specifically, the PTAP (Pro-Thr-Ala-Pro)-type primary L domain of HIV-1 recruits ESCRT-I by binding to Tsg101 (tumour susceptibility gene 101), and an auxiliary LYPX(n)L (Leu-Tyr-Pro-Xaa(n)-Leu)-type L domain recruits the ESCRT-III-binding partner Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X]. The structurally related CHMPs (charged multivesicular body proteins), which form ESCRT-III, are kept in an inactive state through intramolecular interactions, and become potent inhibitors of HIV-1 budding upon removal of an autoinhibitory region. In the absence of the primary L domain, HIV-1 budding is strongly impaired, but can be efficiently rescued through the overexpression of Alix. This effect of Alix depends on its ability to interact with CHMP4, suggesting that it is the recruitment of CHMPs that ultimately drives virus release. Surprisingly, HIV-1 budding defects can also be efficiently corrected by overexpressing Nedd (neural-precursor-cell-expressed developmentally down-regulated) 4-2s, a member of a family of ubiquitin ligases previously implicated in the function of PPXY (Pro-Pro-Xaa-Tyr)-type L domains, which are absent from HIV-1. At least under certain circumstances, Nedd4-2s stimulates the activity of PTAP-type L domains, raising the possibility that the ubiquitin ligase regulates the activity of ESCRT-I.

Piecing together the ESCRTs

High-resolution structural analysis has characterized nearly all of the individual domains of ESCRT (endosomal sorting complex required for transport) subunits, all of the core structures of the soluble complexes and many of the interactions involving domains. Recent emphasis in structural studies has shifted towards efforts to integrate these structures into a larger-scale model. Molecular simulations, hydrodynamic analysis, small-angle X-ray scattering and cryo-EM (electron microscopy) techniques have all been brought to bear on the ESCRT system over the last year.

Regulation of Vps4 ATPase activity by ESCRT-III

MVB (multivesicular body) formation occurs when the limiting membrane of an endosome invaginates into the intraluminal space and buds into the lumen, bringing with it a subset of transmembrane cargoes. Exvagination of the endosomal membrane from the cytosol is topologically similar to the budding of retroviral particles and cytokinesis, wherein membranes bud away from the cytoplasm, and the machinery responsible for MVB sorting has been implicated in these phenomena. The AAA (ATPase associated with various cellular activities) Vps4 (vacuolar protein sorting 4) performs a critical function in the MVB sorting pathway. Vps4 appears to dissociate the ESCRTs (endosomal sorting complexes required for transport) from endosomal membranes during the course of MVB sorting, but it is unclear how Vps4 ATPase activity is synchronized with ESCRT release. We have investigated the mechanisms by which ESCRT components stimulate the ATPase activity of Vps4. These studies support a model wherein Vps4 activity is subject to spatial and temporal regulation via distinct mechanisms during MVB sorting.

Structure and disassembly of filaments formed by the ESCRT-III subunit Vps24

The ESCRT machinery mediates sorting of ubiquitinated transmembrane proteins to lysosomes via multivesicular bodies (MVBs) and also has roles in cytokinesis and viral budding. The ESCRT-III subunits are metastable monomers that transiently assemble on membranes. However, the nature of these assemblies is unknown. Among the core yeast ESCRT-III subunits, Snf7 and Vps24 spontaneously form ordered polymers in vitro. Single-particle EM reconstruction of helical Vps24 filaments shows both parallel and head-to-head subunit arrangements. Mutations of regions involved in intermolecular assembly in vitro result in cargo-sorting defects in vivo, suggesting that these homopolymers mimic interactions formed by ESCRT-III heteropolymers during MVB biogenesis. The C terminus of Vps24 is at the surface of the filaments and is not required for filament assembly. When this region is replaced by the MIT-interacting motif from the Vps2 subunit of ESCRT-III, the AAA-ATPase Vps4 can both bundle and disassemble the chimeric filaments in a nucleotide-dependent fashion.

Midbody targeting of the ESCRT machinery by a noncanonical coiled coil in CEP55

The ESCRT (endosomal sorting complex required for transport) machinery is required for the scission of membrane necks in processes including the budding of HIV-1 and cytokinesis. An essential step in cytokinesis is recruitment of the ESCRT-I complex and the ESCRT-associated protein ALIX to the midbody (the structure that tethers two daughter cells) by the protein CEP55. Biochemical experiments show that peptides from ALIX and the ESCRT-I subunit TSG101 compete for binding to the ESCRT and ALIX-binding region (EABR) of CEP55. We solved the crystal structure of EABR bound to an ALIX peptide at a resolution of 2.0 angstroms. The structure shows that EABR forms an aberrant dimeric parallel coiled coil. Bulky and charged residues at the interface of the two central heptad repeats create asymmetry and a single binding site for an ALIX or TSG101 peptide. Both ALIX and ESCRT-I are required for cytokinesis, which suggests that multiple CEP55 dimers are required for function.

The HIV-1 p6/EIAV p9 docking site in Alix is autoinhibited as revealed by a conformation-sensitive anti-Alix monoclonal antibody

Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X], a component of the endosomal sorting machinery, contains a three-dimensional docking site for HIV-1 p6(Gag) or EIAV (equine infectious anaemia virus) p9(Gag), and binding of the viral protein to this docking site allows the virus to hijack the host endosomal sorting machinery for budding from the plasma membrane. In the present study, we identified a monoclonal antibody that specifically recognizes the docking site for p6(Gag)/p9(Gag) and we used this antibody to probe the accessibility of the docking site in Alix. Our results show that the docking site is not available in cytosolic or recombinant Alix under native conditions and becomes available upon addition of the detergent Nonidet P40 or SDS. In HEK (human embryonic kidney)-293 cell lysates, an active p6(Gag)/p9(Gag) docking site is specifically available in Alix from the membrane fraction. The findings of the present study demonstrate that formation or exposure of the p6(Gag)/p9(Gag) docking site in Alix is a regulated event and that Alix association with the membrane may play a positive role in this process.

Extracellular Alix regulates integrin-mediated cell adhesions and extracellular matrix assembly

Alix (ALG-2-interacting protein X), a cytoplasmic adaptor protein involved in endosomal sorting and actin cytoskeleton assembly, is required for the maintenance of fibroblast morphology. As Alix has sequence similarity to adhesin in Entamoeba histolytica, and we observed that Alix is secreted, we determined whether extracellular Alix affects fibroblast morphology. Here, we demonstrate that secreted Alix is deposited on the substratum of non-immortalized WI38 fibroblasts. Antibody binding to extracellular Alix retards WI38 cell adhesion and spreading on fibronectin and vitronectin. Alix knockdown in WI38 cells reduces spreading and fibronectin assembly, and the effect is partially complemented by coating recombinant Alix on the cell substratum. Immortalized NIH/3T3 fibroblasts deposit less Alix on the substratum and have defects in alpha5beta1-integrin functions. Coating recombinant Alix on the culture substratum for NIH/3T3 cells promotes alpha5beta1-integrin-mediated cell adhesions and fibronectin assembly, and these effects require the aa 605-709 region of Alix. These findings demonstrate that a sub-population of Alix localizes extracellularly and regulates integrin-mediated cell adhesions and fibronectin matrix assembly.

ALIX-CHMP4 interactions in the human ESCRT pathway

The ESCRT pathway facilitates membrane fission events during enveloped virus budding, multivesicular body formation, and cytokinesis. To promote HIV budding and cytokinesis, the ALIX protein must bind and recruit CHMP4 subunits of the ESCRT-III complex, which in turn participate in essential membrane remodeling functions. Here, we report that the Bro1 domain of ALIX binds specifically to C-terminal residues of the human CHMP4 proteins (CHMP4A-C). Crystal structures of the complexes reveal that the CHMP4 C-terminal peptides form amphipathic helices that bind across the conserved concave surface of ALIX(Bro1). ALIX-dependent HIV-1 budding is blocked by mutations in exposed ALIX(Bro1) residues that help contribute to the binding sites for three essential hydrophobic residues that are displayed on one side of the CHMP4 recognition helix (M/L/IxxLxxW). The homologous CHMP1-3 classes of ESCRT-III proteins also have C-terminal amphipathic helices, but, in those cases, the three hydrophobic residues are arrayed with L/I/MxxxLxxL spacing. Thus, the distinct patterns of hydrophobic residues provide a "code" that allows the different ESCRT-III subunits to bind different ESCRT pathway partners, with CHMP1-3 proteins binding MIT domain-containing proteins, such as VPS4 and Vta1/LIP5, and CHMP4 proteins binding Bro1 domain-containing proteins, such as ALIX.

Refined study of the interaction between HIV-1 p6 late domain and ALIX

The interaction between the HIV-1 p6 late budding domain and ALIX, a class E vacuolar protein sorting factor, was explored by using the yeast two-hybrid approach. We refined the ALIX binding site of p6 as being the leucine triplet repeat sequence (Lxx)4 (LYPLTSLRSLFG). Intriguingly, the deletion of the C-terminal proline-rich region of ALIX prevented detectable binding to p6. In contrast, a four-amino acid deletion in the central hinge region of p6 increased its association with ALIX as shown by its ability to bind to ALIX lacking the proline rich domain. Finally, by using a random screening approach, the minimal ALIX391-510 fragment was found to specifically interact with this p6 deletion mutant. A parallel analysis of ALIX binding to the late domain p9 from EIAV revealed that p6 and p9, which exhibit distinct ALIX binding motives, likely bind differently to ALIX. Altogether, our data support a model where the C-terminal proline-rich domain of ALIX allows the access of its binding site to p6 by alleviating a conformational constraint resulting from the presence of the central p6 hinge.

The Bro1-related protein HD-PTP/PTPN23 is required for endosomal cargo sorting and multivesicular body morphogenesis

The Saccharomyces cerevisiae protein Bro1p is required for sorting endocytic cargo to the lumen of multivesicular bodies (MVBs). The mammalian ortholog of Bro1p is not known; although Alix, a structurally related protein, supports the topologically similar process of virus budding, functional studies have so far failed to identify a role for Alix in MVB formation. To establish whether Alix or similar protein(s) participate in endosomal sorting, we attached a retroviral peptide that binds Alix to a reporter receptor. This chimera was sorted efficiently away from the early endosome to the lumen of late endocytic compartments. Surprisingly, sorting was not prevented by depleting Alix but instead required the Alix-related protein His domain phosphotyrosine phosphatase (HD-PTP)/His-Domain/Type N23 protein tyrosine phosphatase (PTPN23). Depletion of HD-PTP also reduced transfer of fluid-phase markers and EGF receptor to lysosomes, caused the accumulation of ubiquitinated proteins on endosomal compartments and disrupted the morphogenesis of MVBs. Rescue experiments using an RNAi-resistant version of HD-PTP and HD-PTP mutants demonstrated an essential role for the HD-PTP Bro1 domain, with ESCRT-III binding correlating with full biological activity.

Myopic acts in the endocytic pathway to enhance signaling by the Drosophila EGF receptor

Endocytosis of activated receptors can control signaling levels by exposing the receptors to novel downstream molecules or by instigating their degradation. Epidermal growth factor receptor (EGFR) signaling has crucial roles in development and is misregulated in many cancers. We report here that Myopic, the Drosophila homolog of the Bro1-domain tyrosine phosphatase HD-PTP, promotes EGFR signaling in vivo and in cultured cells. myopic is not required in the presence of activated Ras or in the absence of the ubiquitin ligase Cbl, indicating that it acts on internalized EGFR, and its overexpression enhances the activity of an activated form of EGFR. Myopic is localized to intracellular vesicles adjacent to Rab5-containing early endosomes, and its absence results in the enlargement of endosomal compartments. Loss of Myopic prevents cleavage of the EGFR cytoplasmic domain, a process controlled by the endocytic regulators Cbl and Sprouty. We suggest that Myopic promotes EGFR signaling by mediating its progression through the endocytic pathway.

ALIX catches HIV

ESCRT and ESCRT-associated proteins are required during the assembly and release of many RNA viruses, including HIV. Two new papers provide structures for the ESCRT-associated protein ALIX/AIP1 and demonstrate how this protein interacts with HIV Gag. One of these studies provides the clearest evidence to date that ESCRT-III mediates key events in virus release and indicates that there are cellular proteins involved in this process still to be discovered.

ESCRT-III family members stimulate Vps4 ATPase activity directly or via Vta1

The AAA-ATPase Vps4 is critical for function of the MVB sorting pathway, which in turn impacts cellular phenomena ranging from receptor downregulation to viral budding to cytokinesis. Vps4 dissociates ESCRTs from endosomal membranes during MVB sorting, but it is unclear how Vps4 ATPase activity is synchronized with ESCRT release. Vta1 potentiates Vps4 activity and interacts with ESCRT-III family members. We have investigated the impact of Vta1 and ESCRT-III family members on Vps4 ATPase activity. Two distinct mechanisms of Vps4 stimulation are described: Vps2 can directly stimulate Vps4 via its MIT domain, whereas Vps60 stimulates via Vta1. Moreover, Did2 can stimulate Vps4 by both mechanisms in distinct contexts. Recent structural determination of the ESCRT-III-binding region of Vta1 unexpectedly revealed a MIT-like region. These data support a model wherein a network of MIT and MIT-like domain interactions with ESCRT-III subunits contributes to the regulation of Vps4 activity during MVB sorting.

Itinerant exosomes: emerging roles in cell and tissue polarity

Cells use secreted signals (e.g. chemokines and growth factors) and sophisticated vehicles such as argosomes, cytonemes, tunneling nanotubes and exosomes to relay important information to other cells, often over large distances. Exosomes, 30-100-nm intraluminal vesicles of multivesicular bodies (MVB) released upon exocytic fusion of the MVB with the plasma membrane, are increasingly recognized as a novel mode of cell-independent communication. Exosomes have been shown to function in antigen presentation and tumor metastasis, and in transmitting infectious agents. However, little is known about the biogenesis and function of exosomes in polarized cells. In this review, we discuss new evidence suggesting that exosomes participate in the transport of morphogens and RNA, and thus influence cell polarity and developmental patterning of tissues.

Brox, a novel farnesylated Bro1 domain-containing protein that associates with charged multivesicular body protein 4 (CHMP4)

Human Brox is a newly identified 46 kDa protein that has a Bro1 domain-like sequence and a C-terminal thioester-linkage site of isoprenoid lipid (CAAX motif) (C standing for cysteine, A for generally aliphatic amino acid, and X for any amino acid). Mammalian Alix and its yeast ortholog, Bro1, are known to associate with charged multivesicular body protein 4 (CHMP4), a component of endosomal sorting complex required for transport III, via their Bro1 domains and to play roles in sorting of ubiquitinated cargoes. We investigated whether Brox has an authentic Bro1 domain on the basis of its capacity for interacting with CHMP4s. Both Strep Tactin binding sequence (Strep)-tagged wild-type Brox (Strep-Brox(WT)) and Strep-tagged farnesylation-defective mutant (Cys-->Ser mutation; Strep-Brox(C408S)) pulled down FLAG-tagged CHMP4b that was coexpressed in HEK293 cells. Treatment of cells with a farnesyltransferase inhibitor, FTI-277, caused an electrophoretic mobility shift of Strep-Brox(WT), and the mobility coincided with that of Strep-Brox(C408S). The inhibitor also caused a mobility shift of endogenous Brox detected by western blotting using polyclonal antibodies to Brox, suggesting farnesylation of Brox in vivo. Fluorescence microscopic analyses revealed that Strep-Brox(WT) exhibited accumulation in the perinuclear area and caused a punctate pattern of FLAG-CHMP4b that was constitutively expressed in HEK293 cells. On the other hand, Strep-Brox(C408S) showed a diffuse pattern throughout the cell, including the nucleus, and did not cause accumulation of FLAG-CHMP4b. Fluorescent signals of monomeric green fluorescent protein (mGFP)-fused Brox(WT) merged partly with those of Golgi markers and with those of abnormal endosomes induced by overexpression of a dominant negative mutant of AAA type ATPase SKD1/Vps4B in HeLa cells, but such colocalization was less efficient for mGFP-Brox(C408S). These results suggest a physiological significance of farnesylation of Brox in its subcellular distribution and efficient interaction with CHMP4s in vivo.

Beyond Tsg101: the role of Alix in 'ESCRTing' HIV-1

The limited coding capacity of retroviral genomes forces these viruses to rely heavily on the host-cell machinery for their replication. This phenomenon is particularly well illustrated by the interaction between retroviruses and components of the endosomal budding machinery that occurs during virus release. Here, we focus on the use of host-cell factors during HIV-1 budding and highlight recent progress in our understanding of the role of one such factor, Alix, in both viral and cellular membrane budding and fission events.

Structural and functional studies of ALIX interactions with YPX(n)L late domains of HIV-1 and EIAV

Retrovirus budding requires short peptide motifs (late domains) located within the viral Gag protein that function by recruiting cellular factors. The YPX(n)L late domains of HIV and other lentiviruses recruit the protein ALIX (also known as AIP1), which also functions in vesicle formation at the multivesicular body and in the abscission stage of cytokinesis. Here, we report the crystal structures of ALIX in complex with the YPX(n)L late domains from HIV-1 and EIAV. The two distinct late domains bind at the same site on the ALIX V domain but adopt different conformations that allow them to make equivalent contacts. Binding studies and functional assays verified the importance of key interface residues and revealed that binding affinities are tuned by context-dependent effects. These results reveal how YPX(n)L late domains recruit ALIX to facilitate virus budding and how ALIX can bind YPX(n)L sequences with both n = 1 and n = 3.

A novel requirement for C. elegans Alix/ALX-1 in RME-1-mediated membrane transport

BACKGROUND

Alix/Bro1p family proteins have recently been identified as important components of multivesicular endosomes (MVEs) and are involved in the sorting of endocytosed integral membrane proteins, interacting with components of the ESCRT complex, the unconventional phospholipid LBPA, and other known endocytosis regulators. During infection, Alix can be co-opted by enveloped retroviruses, including HIV, providing an important function during virus budding from the plasma membrane. In addition, Alix is associated with the actin cytoskeleton and might regulate cytoskeletal dynamics.

RESULTS

Here we demonstrate a novel physical interaction between the only apparent Alix/Bro1p family protein in C. elegans, ALX-1, and a key regulator of receptor recycling from endosomes to the plasma membrane, called RME-1. The analysis of alx-1 mutants indicates that ALX-1 is required for the endocytic recycling of specific basolateral cargo in the C. elegans intestine, a pathway previously defined by the analysis of rme-1 mutants. The expression of truncated human Alix in HeLa cells disrupts the recycling of major histocompatibility complex class I, a known Ehd1/RME-1-dependent transport step, suggesting the phylogenetic conservation of this function. We show that the interaction of ALX-1 with RME-1 in C. elegans, mediated by RME-1/YPSL and ALX-1/NPF motifs, is required for this recycling process. In the C. elegans intestine, ALX-1 localizes to both recycling endosomes and MVEs, but the ALX-1/RME-1 interaction appears to be dispensable for ALX-1 function in MVEs and/or late endosomes.

CONCLUSIONS

This work provides the first demonstration of a requirement for an Alix/Bro1p family member in the endocytic recycling pathway in association with the recycling regulator RME-1.

PalC, one of two Bro1 domain proteins in the fungal pH signalling pathway, localizes to cortical structures and binds Vps32

PalC, distantly related to Saccharomyces cerevisiaeperipheral endosomal sorting complexes required for transport III (ESCRT-III) component Bro1p and one of six Aspergillus nidulanspH signalling proteins, contains a Bro1 domain. Green fluorescent protein (GFP)-tagged PalC is recruited to plasma membrane-associated punctate structures upon alkalinization, when pH signalling is active. PalC recruitment to these structures is dependent on the seven transmembrane domain (7-TMD) receptor and likely pH sensor PalH. PalC is a two-hybrid interactor of the ESCRT-III Vps20/Vps32 subcomplex and binds Vps32 directly. This binding is largely impaired by Pro439Phe, Arg442Ala and Arg442His substitutions in a conserved region mediating interaction of Bro1p with Vps32p, but these substitutions do not prevent cortical punctate localization, indicating Vps32 independence. In contrast, Arg442Δ impairs Vps32 binding and prevents PalC-GFP recruitment to cortical structures. pH signalling involves a plasma membrane complex including the 7-TMD receptor PalH and the arrestin-like PalF and an endosomal membrane complex involving the PalB protease, the transcription factor PacC and the Vps32 binding, Bro1-domain-containing protein PalA. PalC, which localizes to cortical structures and can additionally bind a component of ESCRT-III, has the features required to bridge these two entities. A likely S. cerevisiaeorthologue of PalC has been identified, providing the basis for a unifying hypothesis of gene regulation by ambient pH in ascomycetes.

Split-ubiquitin two-hybrid assay to analyze protein-protein interactions at the endosome: application to Saccharomyces cerevisiae Bro1 interacting with ESCRT complexes, the Doa4 ubiquitin hydrolase, and the Rsp5 ubiquitin ligase

Targeting of membrane proteins into the lysosomal/vacuolar lumen for degradation requires their prior sorting into multivesicular bodies (MVB). The MVB sorting pathway depends on ESCRT-0, -I, -II, and -III protein complexes functioning on the endosomal membrane and on additional factors, such as Bro1/Alix and the ubiquitin ligase Rsp5/Nedd4. We used the split-ubiquitin two-hybrid assay to analyze the interaction partners of yeast Bro1 at its natural cellular location. We show that Bro1 interacts with ESCRT-I and -III components, including Vps23, the Saccharomyces cerevisiae homologue of human Tsg101. These interactions do not require the C-terminal proline-rich domain (PRD) of Bro1. Rather, this PRD interacts with the Doa4 deubiquitinating enzyme to recruit it to the endosome. This interaction is disrupted by a single amino acid substitution in the conserved ELC box motif in Doa4. The PRD of Bro1 also mediates an association with Rsp5, and this interaction appears to be conserved, as Alix, the human homologue of Bro1, coimmunoprecipitates with Nedd4 in yeast lysates. We further show that the Bro1 PRD domain is essential to MVB sorting of only cargo proteins whose sorting to the vacuolar lumen is dependent on their own ubiquitination and Doa4. The Bro1 region preceding the PRD, however, is required for MVB sorting of proteins irrespective of whether their targeting to the vacuole is dependent on their ubiquitination and Doa4. Our data indicate that Bro1 interacts with several ESCRT components and contributes via its PRD to associating ubiquitinating and deubiquitinating enzymes with the MVB sorting machinery.

The Bro1-domain protein, EGO-2, promotes Notch signaling in Caenorhabditis elegans

In Caenorhabditis elegans, as in other animals, Notch-type signaling mediates numerous inductive events during development. The mechanism of Notch-type signaling involves proteolytic cleavage of the receptor and subsequent transport of the receptor intracellular domain to the nucleus, where it acts as a transcriptional regulator. Notch-type signaling activity is modulated by post-translational modifications and endocytosis of ligand and receptor. We previously identified the ego-2 (enhancer of glp-1) gene as a positive regulator of germline proliferation that interacts genetically with the GLP-1/Notch signaling pathway in the germline. Here, we show that ego-2 positively regulates signaling in various tissues via both GLP-1 and the second C. elegans Notch-type receptor, LIN-12. ego-2 activity also promotes aspects of development not known to require GLP-1 or LIN-12. The EGO-2 protein contains a Bro1 domain, which is known in other systems to localize to certain endosomal compartments. EGO-2 activity in the soma promotes GLP-1 signaling in the germline, consistent with a role for EGO-2 in production of active ligand. Another C. elegans Bro1-domain protein, ALX-1, is known to interact physically with LIN-12/Notch. We document a complex phenotypic interaction between ego-2 and alx-1, consistent with their relationship being antagonistic with respect to some developmental processes and agonistic with respect to others.

Trafficking and developmental signaling: Alix at the crossroads

Alix is a phylogenetically conserved protein that participates in mammals in programmed cell death in association with ALG-2, a penta-EF-hand calciprotein. It contains an N-terminal Bro1 domain, a coiled-coil region and a C-terminal proline-rich domain containing several SH3- and WW-binding sites that contribute to its scaffolding properties. Recent data showed that by virtue of its Bro1 domain, Alix is functionally associated to the ESCRT complexes involved in the biogenesis of the multivesicular body and sorting of transmembrane proteins within this specific endosomal compartment. In Dictyostelium, an alx null strain shows a markedly perturbed starvation-induced morphogenetic program while ALG-2 disruptants remain unaffected. This review summarizes Dictyostelium data on Alix and ALG-2 homologues and evaluates whether known functions of Alix in other organisms can account for the developmental arrest of the alx null mutant and how Dictyostelium studies can substantiate the current understanding of the function(s) of this versatile and conserved signaling molecule.

The emerging shape of the ESCRT machinery

The past two years have seen an explosion in the structural understanding of the endosomal sorting complex required for transport (ESCRT) machinery that facilitates the trafficking of ubiquitylated proteins from endosomes to lysosomes via multivesicular bodies (MVBs). A common organization of all ESCRTs is a rigid core attached to flexibly connected modules that recognize other components of the MVB pathway. Several previously unsuspected key links between multiple ESCRT subunits, phospholipids and ubiquitin have now been elucidated, which, together with the detailed morphological analyses of ESCRT-depletion phenotypes, provide new insights into the mechanism of MVB biogenesis.

Membrane Curvature: The Power of Bananas, Zeppelins and Boomerangs

New work on the ability of IRSp53/MIM domains to induce negative membrane curvature sheds light on the mechanisms used to generate actin-rich cell protrusions.

The multiple personalities of Alix

Alix is a cytosolic protein in mammalian cells that was originally identified on the basis of its association with pro-apoptotic signaling. More recent evidence has established that Alix has a hand in regulating other cellular mechanisms, including endocytic membrane trafficking and cell adhesion. Although Alix appears to participate directly in these various activities, the role it plays in each process has largely been inferred from the functions of proteins with which it interacts. For example, recruitment of Alix to endosomes is mediated by its N-terminal Bro1 domain, the structure of which was recently solved for its yeast orthologue, Bro1. The diversity of Alix functions is due to its proline-rich C-terminus, which provides multiple protein-binding sites. With this blueprint in hand, we can now ask whether Alix acts simply as an adaptor that links different proteins into networks or, instead, contributes a specific function to distinct molecular machineries.

Molecular architecture and functional model of the complete yeast ESCRT-I heterotetramer

The endosomal sorting complex required for transport-I (ESCRT-I) complex, which is conserved from yeast to humans, directs the lysosomal degradation of ubiquitinated transmembrane proteins and the budding of the HIV virus. Yeast ESCRT-I contains four subunits, Vps23, Vps28, Vps37, and Mvb12. The crystal structure of the heterotetrameric ESCRT-I complex reveals a highly asymmetric complex of 1:1:1:1 subunit stoichiometry. The core complex is nearly 18 nm long and consists of a headpiece attached to a 13 nm stalk. The stalk is important for cargo sorting by ESCRT-I and is proposed to serve as a spacer regulating the correct disposition of cargo and other ESCRT components. Hydrodynamic constraints and crystallographic structures were used to generate a model of intact ESCRT-I in solution. The results show how ESCRT-I uses a combination of a rigid stalk and flexible tethers to interact with lipids, cargo, and other ESCRT complexes over a span of approximately 25 nm.

Dual mechanisms specify Doa4-mediated deubiquitination at multivesicular bodies

Doa4 is a ubiquitin-specific protease in Saccharomyces cerevisiae that deubiquitinates integral membrane proteins sorted into the lumenal vesicles of late-endosomal multivesicular bodies (MVBs). We show that the non-catalytic N terminus of Doa4 mediates its recruitment to endosomes through its association with Bro1, which is one of several highly conserved class E Vps proteins that comprise the core MVB sorting machinery. In turn, Bro1 directly stimulates deubiquitination by interacting with a YPxL motif in the catalytic domain of Doa4. Mutations in either Doa4 or Bro1 that disrupt catalytic activation of Doa4 impair deubiquitination and sorting of MVB cargo proteins and lead to the formation of lumenal MVB vesicles that are predominantly small compared with the vesicles seen in wild-type cells. Thus, by recruiting Doa4 to late endosomes and stimulating its catalytic activity, Bro1 fulfills a novel dual role in coordinating deubiquitination in the MVB pathway.

Structural and biochemical studies of ALIX/AIP1 and its role in retrovirus budding

ALIX/AIP1 functions in enveloped virus budding, endosomal protein sorting, and many other cellular processes. Retroviruses, including HIV-1, SIV, and EIAV, bind and recruit ALIX through YPX(n)L late-domain motifs (X = any residue; n = 1-3). Crystal structures reveal that human ALIX is composed of an N-terminal Bro1 domain and a central domain that is composed of two extended three-helix bundles that form elongated arms that fold back into a "V." The structures also reveal conformational flexibility in the arms that suggests that the V domain may act as a flexible hinge in response to ligand binding. YPX(n)L late domains bind in a conserved hydrophobic pocket on the second arm near the apex of the V, whereas CHMP4/ESCRT-III proteins bind a conserved hydrophobic patch on the Bro1 domain, and both interactions are required for virus budding. ALIX therefore serves as a flexible, extended scaffold that connects retroviral Gag proteins to ESCRT-III and other cellular-budding machinery.

Potent rescue of human immunodeficiency virus type 1 late domain mutants by ALIX/AIP1 depends on its CHMP4 binding site

The release of human immunodeficiency virus type 1 (HIV-1) and of other retroviruses from certain cells requires the presence of distinct regions in Gag that have been termed late assembly (L) domains. HIV-1 harbors a PTAP-type L domain in the p6 region of Gag that engages an endosomal budding machinery through Tsg101. In addition, an auxiliary L domain near the C terminus of p6 binds to ALIX/AIP1, which functions in the same endosomal sorting pathway as Tsg101. In the present study, we show that the profound release defect of HIV-1 L domain mutants can be completely rescued by increasing the cellular expression levels of ALIX and that this rescue depends on an intact ALIX binding site in p6. Furthermore, the ability of ALIX to rescue viral budding in this system depended on two putative surface-exposed hydrophobic patches on its N-terminal Bro1 domain. One of these patches mediates the interaction between ALIX and the ESCRT-III component CHMP4B, and mutations which disrupt the interaction also abolish the activity of ALIX in viral budding. The ability of ALIX to rescue a PTAP mutant also depends on its C-terminal proline-rich domain (PRD), but not on the binding sites for Tsg101, endophilin, CIN85, or for the newly identified binding partner, CMS, within the PRD. Our data establish that ALIX can have a dramatic effect on HIV-1 release and suggest that the ability to use ALIX may allow HIV-1 to replicate in cells that express only low levels of Tsg101.

Further characterization of the signaling proteolysis step in the Aspergillus nidulans pH signal transduction pathway

The Aspergillus nidulans pH-responsive transcription factor PacC is modulated by limited, two-step proteolysis. The first, pH-regulated cleavage occurs in the 24-residue highly conserved "signaling protease box" in response to the alkaline pH signal. This is transduced by the Pal signaling pathway, containing the predicted calpain-like cysteine protease and likely signaling protease, PalB. In this work, we carried out classical mutational analysis of the putative signaling protease PalB, and we describe 9 missense and 18 truncating loss-of-function (including null) mutations. Mutations in the region of and affecting directly the predicted catalytic cysteine strongly support the deduction that PalB is a cysteine protease. Truncating and missense mutations affecting the C terminus highlight the importance of this region. Analysis of three-hemagglutinin-tagged PalB in Western blots demonstrates that PalB levels are independent of pH and Pal signal transduction. We have followed the processing of MYC(3)-tagged PacC in Western blots. We show unequivocally that PalB is essential for signaling proteolysis and is definitely not the processing protease. In addition, we have replaced 15 residues of the signaling protease box of MYC(3)-tagged PacC (pacC900) with alanine. The majority of these substitutions are silent. Leu481Ala, Tyr493Ala, and Gln499Ala result in delayed PacC processing in response to shifting from acidic to alkaline medium, as determined by Western blot analysis. Leu498Ala reduces function much more markedly, as determined by plate tests and processing recalcitrance. Excepting Leu498, this demonstrates that PacC signaling proteolysis is largely independent of sequence in the cleavage region.

New component of ESCRT-I regulates endosomal sorting complex assembly

The endosomal sorting complex required for transport (ESCRT) complexes play a critical role in receptor down-regulation and retroviral budding. Although the crystal structures of two ESCRT complexes have been determined, the molecular mechanisms underlying the assembly and regulation of the ESCRT machinery are still poorly understood. We identify a new component of the ESCRT-I complex, multivesicular body sorting factor of 12 kD (Mvb12), and demonstrate that Mvb12 binds to the coiled-coil domain of the ESCRT-I subunit vacuolar protein sorting 23 (Vps23). We show that ESCRT-I adopts an oligomeric state in the cytosol, the formation of which requires the coiled-coil domain of Vps23, as well as Mvb12. Loss of Mvb12 results in the disassembly of the ESCRT-I oligomer and the formation of a stable complex of ESCRT-I and -II in the cytosol. We propose that Mvb12 stabilizes ESCRT-I in an oligomeric, inactive state in the cytosol to ensure that the ordered recruitment and assembly of ESCRT-I and -II is spatially and temporally restricted to the surface of the endosome after activation of the MVB sorting reaction.

A conserved late endosome-targeting signal required for Doa4 deubiquitylating enzyme function

Enzyme specificity in vivo is often controlled by subcellular localization. Yeast Doa4, a deubiquitylating enzyme (DUB), removes ubiquitin from membrane proteins destined for vacuolar degradation. Doa4 is recruited to the late endosome after ESCRT-III (endosomal sorting complex required for transport III) has assembled there. We show that an N-terminal segment of Doa4 is sufficient for endosome association. This domain bears four conserved elements (boxes A–D). Deletion of the most conserved of these, A or B, prevents Doa4 endosomal localization. These mutants cannot sustain ubiquitin-dependent proteolysis even though neither motif is essential for deubiquitylating activity. Ubiquitin-specific processing protease 5 (Ubp5), the closest paralogue of Doa4, has no functional overlap. Ubp5 concentrates at the bud neck; its N-terminal domain is critical for this. Importantly, substitution of the Ubp5 N-terminal domain with that of Doa4 relocalizes the Ubp5 enzyme to endosomes and provides Doa4 function. This is the first demonstration of a physiologically important DUB subcellular localization signal and provides a striking example of the functional diversification of DUB paralogues by the evolution of alternative spatial signals.

Structural basis for viral late-domain binding to Alix

The modular protein Alix is a central node in endosomal-lysosomal trafficking and the budding of human immunodeficiency virus (HIV)-1. The Gag p6 protein of HIV-1 contains a LYPx(n)LxxL motif that is required for Alix-mediated budding and binds a region of Alix spanning residues 360-702. The structure of this fragment of Alix has the shape of the letter 'V' and is termed the V domain. The V domain has a topologically complex arrangement of 11 alpha-helices, with connecting loops that cross three times between the two arms of the V. The conserved residue Phe676 is at the center of a large hydrophobic pocket and is crucial for binding to a peptide model of HIV-1 p6. Overexpression of the V domain inhibits HIV-1 release from cells. This inhibition of release is reversed by mutations that block binding of the Alix V domain to p6.

Phosphorylation of the proline-rich domain of Xp95 modulates Xp95 interaction with partner proteins

The mammalian adaptor protein Alix [ALG-2 (apoptosis-linked-gene-2 product)-interacting protein X] belongs to a conserved family of proteins that have in common an N-terminal Bro1 domain and a C-terminal PRD (proline-rich domain), both of which mediate partner protein interactions. Following our previous finding that Xp95, the Xenopus orthologue of Alix, undergoes a phosphorylation-dependent gel mobility shift during progesteroneinduced oocyte meiotic maturation, we explored potential regulation of Xp95/Alix by protein phosphorylation in hormone-induced cell cycle re-entry or M-phase induction. By MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analyses and gel mobility-shift assays, Xp95 is phosphorylated at multiple sites within the N-terminal half of the PRD during Xenopus oocyte maturation, and a similar region in Alix is phosphorylated in mitotically arrested but not serum-stimulated mammalian cells. By tandem MS, Thr745 within this region, which localizes in a conserved binding site to the adaptor protein SETA [SH3 (Src homology 3) domain-containing, expressed in tumorigenic astrocytes] CIN85 (a-cyano-4-hydroxycinnamate)/SH3KBP1 (SH3-domain kinase-binding protein 1), is one of the phosphorylation sites in Xp95. Results from GST (glutathione S-transferase)-pull down and peptide binding/competition assays further demonstrate that the Thr745 phosphorylation inhibits Xp95 interaction with the second SH3 domain of SETA. However, immunoprecipitates of Xp95 from extracts of M-phase-arrested mature oocytes contained additional partner proteins as compared with immunoprecipitates from extracts of G2-arrested immature oocytes. The deubiquitinase AMSH (associated molecule with the SH3 domain of signal transducing adaptor molecule) specifically interacts with phosphorylated Xp95 in M-phase cell lysates. These findings establish that Xp95/Alix is phosphorylated within the PRD during M-phase induction, and indicate that the phosphorylation may both positively and negatively modulate their interaction with partner proteins.

An Alix fragment potently inhibits HIV-1 budding: characterization of binding to retroviral YPXL late domains

The retroviral structural protein, Gag, contains small peptide motifs known as late domains that promote efficient virus release from the infected cell. In addition to the well characterized PTAP late domain, the p6 region of HIV-1 Gag contains a binding site for the host cell protein Alix. To better understand the functional role of the Gag/Alix interaction, we overexpressed an Alix fragment composed of residues 364-716 (Alix 364-716) and examined the effect on release of wild type (WT) and Alix binding site mutant HIV-1. We observed that Alix 364-716 expression significantly inhibited WT virus release and Gag processing and that mutation of the Alix binding site largely relieved this inhibition. Furthermore, Alix 364-716 expression induced a severe defect on WT but not mutant particle morphology. Intriguingly, the impact of Alix 364-716 expression on HIV-1 release and Gag processing was markedly different from that induced by mutation of the Alix binding site in p6. The association of Alix 364-716 with HIV-1 and equine infectious anemia virus late domains was quantitatively evaluated by isothermal titration calorimetry and surface plasmon resonance techniques, and the effects of mutations in these viral sequences on Alix 364-716 binding was determined. This study identifies a novel Alix-derived dominant negative inhibitor of HIV-1 release and Gag processing and provides quantitative information on the interaction between Alix and viral late domains.

Rhophilin-2 is targeted to late-endosomal structures of the vesicular machinery in the presence of activated RhoB

Rhophilin-2 or p76(RBE), a protein whose expression is induced by the cyclic AMP pathway in thyrocytes, contains several protein-protein interaction domains including HR-1, Bro1 and PDZ domains, and is a partner of RhoB in its GTP-bound form (Eur J Biochem, 269(24): 6241-9, 2002). We here define its subcellular localization and dissect the significance of its domains. By subcellular fractionation and colocalization experiments, rhophilin-2 is recruited to subcellular organelles by activated RhoB-GTP. As for its yeast homologue, Npi3/Bro1p, the Bro1 domain of rhophilin-2 is necessary to its recruitment to the vesicular structures, which are not labeled for EEA1 nor Lamp1, but well with the late endosome marker CD63.

Did2 coordinates Vps4-mediated dissociation of ESCRT-III from endosomes

The sorting of transmembrane cargo proteins into the lumenal vesicles of multivesicular bodies (MVBs) depends on the recruitment of endosomal sorting complexes required for transport (ESCRTs) to the cytosolic face of endosomal membranes. The subsequent dissociation of ESCRT complexes from endosomes requires Vps4, a member of the AAA family of adenosine triphosphatases. We show that Did2 directs Vps4 activity to the dissociation of ESCRT-III but has no role in the dissociation of ESCRT-I or -II. Surprisingly, vesicle budding into the endosome lumen occurs in the absence of Did2 function even though Did2 is required for the efficient sorting of MVB cargo proteins into lumenal vesicles. This uncoupling of MVB cargo sorting and lumenal vesicle formation suggests that the Vps4-mediated dissociation of ESCRT-III is an essential step in the sorting of cargo proteins into MVB vesicles but is not a prerequisite for the budding of vesicles into the endosome lumen.

HD-PTP and Alix share some membrane-traffic related proteins that interact with their Bro1 domains or proline-rich regions

Mammalian Alix is a multifunctional adaptor protein involved in cell death, receptor endocytosis, endosomal protein sorting and cell adhesion by associating with various proteins such as ALG-2, CIN85/Rukl/SETA, endophilins, CHMP4s and TSG101. HD-PTP is a paralog of Alix and a putative protein tyrosine phosphatase (PTP) that contains a Bro1 domain, coiled-coils, a proline-rich region (PRR) in addition to a PTP domain. We investigated interactions between HD-PTP and Alix-binding proteins. In the yeast two-hybrid assay, HD-PTP showed positive interactions with CHMP4b/Shax1, TSG101, endophilin A1 and ALG-2 but not with either RabGAPLP or CIN85. We confirmed the interactions in a mammalian system by Strep-pulldown assays in which pulldown products from the lysates of HEK293T cells expressing either Strep-tagged HD-PTP alone or co-expressing with epitope-tagged proteins were analyzed by Western blotting using specific antibodies. While Alix associated with both ALG-2 and TSG101 in a Ca2+-dependent manner, HD-PTP interacted with ALG-2 Ca2+-dependently but with TSG101 Ca2+-independently.

Human ESCRT-II complex and its role in human immunodeficiency virus type 1 release

The budding of many enveloped RNA viruses, including human immunodeficiency virus type 1 (HIV-1), requires some of the same cellular machinery as vesicle formation at the multivesicular body (MVB). In Saccharomyces cerevisiae, the ESCRT-II complex performs a central role in MVB protein sorting and vesicle formation, as it is recruited by the upstream ESCRT-I complex and nucleates assembly of the downstream ESCRT-III complex. Here, we report that the three subunits of human ESCRT-II, EAP20, EAP30, and EAP45, have a number of properties in common with their yeast orthologs. Specifically, EAP45 bound ubiquitin via its N-terminal GRAM-like ubiquitin-binding in EAP45 (GLUE) domain, both EAP45 and EAP30 bound the C-terminal domain of TSG101/ESCRT-I, and EAP20 bound the N-terminal half of CHMP6/ESCRT-III. Consistent with its expected role in MVB vesicle formation, (i) human ESCRT-II localized to endosomal membranes in a VPS4-dependent fashion and (ii) depletion of EAP20/ESCRT-II and CHMP6/ESCRT-III inhibited lysosomal targeting and downregulation of the epidermal growth factor receptor, albeit to a lesser extent than depletion of TSG101/ESCRT-I. Nevertheless, HIV-1 release and infectivity were not reduced by efficient small interfering RNA depletion of EAP20/ESCRT-II or CHMP6/ESCRT-III. These observations indicate that there are probably multiple pathways for protein sorting/MVB vesicle formation in human cells and that HIV-1 does not utilize an ESCRT-II-dependent pathway to leave the cell.

Involvement of the conserved adaptor protein Alix in actin cytoskeleton assembly

The conserved adaptor protein Alix, also called AIP1 or Hp95, promotes flattening and alignment of cultured mammalian fibroblasts; however, the mechanism by which Alix regulates fibroblast morphology is not understood. Here we demonstrate that Alix in WI38 cells, which require Alix expression for maintaining typical fibroblast morphology, associates with filamentous actin (F-actin) and F-actin-based structures lamellipodia and stress fibers. Reducing Alix expression by small interfering RNA (siRNA) decreases F-actin content and inhibits stress fiber assembly. In cell-free systems, Alix directly interacts with F-actin at both the N-terminal Bro1 domain and the C-terminal proline-rich domain. In Alix immunoprecipitates from WI38 cell lysates, actin is the most abundant partner protein of Alix. In addition, the N-terminal half of the middle region of Alix binds cortactin, an activator of the ARP2/3 complex-mediated initiation of actin polymerization. Alix is required for lamellipodial localization of cortactin. The C-terminal half of the middle region of Alix interacts with alpha-actinin, a key factor that bundles F-actin in stress fibers. Alix knockdown decreases the amount of alpha-actinin that associates with F-actin. These findings establish crucial involvement of Alix in actin cytoskeleton assembly.

The ESCRT complexes: structure and mechanism of a membrane-trafficking network

The ESCRT complexes and associated proteins comprise a major pathway for the lysosomal degradation of transmembrane proteins and are critical for receptor downregulation, budding of the HIV virus, and other normal and pathological cell processes. The ESCRT system is conserved from yeast to humans. The ESCRT complexes form a network that recruits monoubiquitinated proteins and drives their internalization into lumenal vesicles within a type of endosome known as a multivesicular body. The structures and interactions of many of the components have been determined over the past three years, revealing mechanisms for membrane and cargo recruitment and for complex assembly.

Molecular mechanisms of late endosome morphology, identity and sorting

Recent studies using electron microscopy, protein crystallography, classic biochemistry and novel live-cell imaging have provided numerous insights into the endocytic pathway, describing a dynamic system in which compartment morphology, molecular identity and the mechanics of cargo sorting are intimately connected. Current evidence supports a model of maturation in which the lipids, cargo proteins and Rab population at the endosome determine its competence to perform the functions of late endosomes, including the sorting of cargoes into lumenal vesicles and fusion with lysosomes.

The crystal structure of the C-terminal domain of Vps28 reveals a conserved surface required for Vps20 recruitment

The endosomal sorting complex I required for transport (ESCRT-I) is composed of the three subunits Vps23/Tsg101, Vps28 and Vps37. ESCRT-I is recruited to cellular membranes during multivesicular endosome biogenesis and by enveloped viruses such as HIV-1 to mediate budding from the cell. Here, we describe the crystal structure of a conserved C-terminal domain from Sacharomyces cerevisiae Vps28 (Vps28-CTD) at 3.05 A resolution which folds independently into a four-helical bundle structure. Co-expression experiments of Vps28-CTD, Vps23 and Vps37 suggest that Vps28-CTD does not directly participate in ESCRT-I assembly and may thus act as an adaptor module for downstream interaction partners. We show through mutagenesis studies that Vps28-CTD employs its strictly conserved surface in the interaction with the ESCRT-III factor Vps20. Furthermore, we present evidence that Vps28-CTD is sufficient to rescue an equine infectious anaemia virus (EIAV) Gag late domain deletion. Vps28-CTD mutations abolishing Vps20 interaction in vitro also prevent the rescue of the EIAV Gag late domain mutant consistent with a potential direct Vps28-ESCRT-III Vps20 recruitment. Therefore, the physiological relevant EIAV Gag-Alix interaction can be functionally replaced by a Gag-Vps28-CTD fusion. Because both Alix and Vps28-CTD can directly recruit ESCRT-III proteins, ESCRT-III assembly coupled to Vps4 action may therefore constitute the minimal budding machinery for EIAV release.

ESCRT-I core and ESCRT-II GLUE domain structures reveal role for GLUE in linking to ESCRT-I and membranes

ESCRT complexes form the main machinery driving protein sorting from endosomes to lysosomes. Currently, the picture regarding assembly of ESCRTs on endosomes is incomplete. The structure of the conserved heterotrimeric ESCRT-I core presented here shows a fan-like arrangement of three helical hairpins, each corresponding to a different subunit. Vps23/Tsg101 is the central hairpin sandwiched between the other subunits, explaining the critical role of its "steadiness box" in the stability of ESCRT-I. We show that yeast ESCRT-I links directly to ESCRT-II, through a tight interaction of Vps28 (ESCRT-I) with the yeast-specific zinc-finger insertion within the GLUE domain of Vps36 (ESCRT-II). The crystal structure of the GLUE domain missing this insertion reveals it is a split PH domain, with a noncanonical lipid binding pocket that binds PtdIns3P. The simultaneous and reinforcing interactions of ESCRT-II GLUE domain with membranes, ESCRT-I, and ubiquitin are critical for ubiquitinated cargo progression from early to late endosomes.

Alix, making a link between apoptosis-linked gene-2, the endosomal sorting complexes required for transport, and neuronal death in vivo

Alix/apoptosis-linked gene-2 (ALG-2)-interacting protein X is an adaptor protein involved in the regulation of the endolysosomal system through binding to endophilins and to endosomal sorting complexes required for transport (ESCRT) proteins, TSG101 and CHMP4b. It was first characterized as an interactor of ALG-2, a calcium-binding protein necessary for cell death, and several observations suggest a role for Alix in controlling cell death. We used electroporation in the chick embryo to test whether overexpressed wild-type or mutated Alix proteins influence cell death in vivo. We show that Alix overexpression is sufficient to induce cell death of neuroepithelial cells. This effect is strictly dependent on its capacity to bind to ALG-2. On the other hand, expression of Alix mutants lacking the ALG-2 or the CHMP4b binding sites prevents early programmed cell death in cervical motoneurons at day 4.5 of chick embryo development. This protection afforded by Alix mutants was abolished after deletion of the TSG101, but not of the endophilin, binding sites. Our results suggest that the interaction of the ALG-2/Alix complex with ESCRT proteins is necessary for naturally occurring death of motoneurons. Therefore, Alix represents a molecular link between the endolysosomal system and the cell death machinery.

Control of Bro1-domain protein Rim20 localization by external pH, ESCRT machinery, and the Saccharomyces cerevisiae Rim101 pathway

Bro1-domain proteins such as yeast Bro1 and mammalian AIP1/Alix are well-established participants in endosome metabolism. The Bro1-domain interacts with endosomal surface protein Snf7/Vps32 in yeast, a subunit of the ESCRT complex. Yeast Bro1-domain protein Rim20 has no role in endosome function, but is required for alkaline pH-stimulated cleavage of transcription factor Rim101. Rim20-GFP is cytoplasmic under acidic conditions but concentrated in punctate foci under alkaline conditions. Bro1-GFP also accumulates in foci, but they are more numerous under acidic than alkaline conditions. Colocalization experiments indicate that some Rim20-GFP foci correspond to Bro1-RFP foci, whereas others do not. Rim8, Rim9, Rim21, Dfg16, Snf7, Vps20, Vps23, and Vps25, which are required for Rim101 cleavage, are required for appearance of Rim20-GFP foci. ESCRT complexes accumulate on endosome-derived compartments in cells that lack the AAA-ATPase Vps4. We find that Rim20-GFP foci accumulate in a vps4 mutant background independently of external pH, Rim101 pathway-specific genes, and most ESCRT subunit genes except for SNF7. Rim20-GFP foci seem to represent endosomes, because they colocalize with Snf7-RFP and because they correspond to a perivacuolar compartment in the vps4 strain. We propose that alkaline growth conditions alter the endosomal surface to favor Rim20-Snf7 interaction and Rim101 cleavage. Our findings raise the possibility that Bro1-domain proteins may be differentially regulated in the same cell, thereby coupling endosome metabolism to signaling.

Membrane curvature and mechanisms of dynamic cell membrane remodelling

Membrane curvature is no longer seen as a passive consequence of cellular activity but an active means to create membrane domains and to organize centres for membrane trafficking. Curvature can be dynamically modulated by changes in lipid composition, the oligomerization of curvature scaffolding proteins and the reversible insertion of protein regions that act like wedges in membranes. There is an interplay between curvature-generating and curvature-sensing proteins during vesicle budding. This is seen during vesicle budding and in the formation of microenvironments. On a larger scale, membrane curvature is a prime player in growth, division and movement.