Consecutive functions of small GTPases guide HOPS-mediated tethering of late endosomes and lysosomes.

The transfer of endocytosed cargoes to lysosomes (LYSs) requires HOPS, a multiprotein complex that tethers late endosomes (LEs) to LYSs before fusion. Many proteins interact with HOPS on LEs/LYSs. However, it is not clear whether these HOPS interactors localize to LEs or LYSs or how they participate in tethering. Here, we biochemically characterized endosomes purified from untreated or experimentally manipulated cells to put HOPS and interacting proteins in order and to establish their functional interdependence. Our results assign Rab2a and Rab7 to LEs and Arl8 and BORC to LYSs and show that HOPS drives LE-LYS fusion by bridging late endosomal Rab2a with lysosomal BORC-anchored Arl8. We further show that Rab7 is absent from sites of HOPS-dependent tethering but promotes fusion by moving LEs toward LYSs via dynein. Thus, our study identifies the topology of the machinery for LE-LYS tethering and elucidates the role of different small GTPases in the process.

PI4P and BLOC-1 remodel endosomal membranes into tubules.

Intracellular trafficking is mediated by transport carriers that originate by membrane remodeling from donor organelles. Tubular carriers contribute to the flux of membrane lipids and proteins to acceptor organelles, but how lipids and proteins impose a tubular geometry on the carriers is incompletely understood. Using imaging approaches on cells and in vitro membrane systems, we show that phosphatidylinositol-4-phosphate (PI4P) and biogenesis of lysosome-related organelles complex 1 (BLOC-1) govern the formation, stability, and functions of recycling endosomal tubules. In vitro, BLOC-1 binds and tubulates negatively charged membranes, including those containing PI4P. In cells, endosomal PI4P production by type II PI4-kinases is needed to form and stabilize BLOC-1-dependent recycling endosomal tubules. Decreased PI4KIIs expression impairs the recycling of endosomal cargoes and the life cycles of intracellular pathogens such as Chlamydia bacteria and influenza virus that exploit the membrane dynamics of recycling endosomes. This study demonstrates how a phospholipid and a protein complex coordinate the remodeling of cellular membranes into functional tubules.

RUFY3 and RUFY4 are ARL8 effectors that promote coupling of endolysosomes to dynein-dynactin.

The small GTPase ARL8 associates with endolysosomes, leading to the recruitment of several effectors that couple endolysosomes to kinesins for anterograde transport along microtubules, and to tethering factors for eventual fusion with other organelles. Herein we report the identification of the RUN- and FYVE-domain-containing proteins RUFY3 and RUFY4 as ARL8 effectors that promote coupling of endolysosomes to dynein-dynactin for retrograde transport along microtubules. Using various methodologies, we find that RUFY3 and RUFY4 interact with both GTP-bound ARL8 and dynein-dynactin. In addition, we show that RUFY3 and RUFY4 promote concentration of endolysosomes in the juxtanuclear area of non-neuronal cells, and drive redistribution of endolysosomes from the axon to the soma in hippocampal neurons. The function of RUFY3 in retrograde transport contributes to the juxtanuclear redistribution of endolysosomes upon cytosol alkalinization. These studies thus identify RUFY3 and RUFY4 as ARL8-dependent, dynein-dynactin adaptors or regulators, and highlight the role of ARL8 in the control of both anterograde and retrograde endolysosome transport.

ARL8 Relieves SKIP Autoinhibition to Enable Coupling of Lysosomes to Kinesin-1.

Long-range movement of organelles within the cytoplasm relies on coupling to microtubule motors, a process that is often mediated by adaptor proteins. In many cases, this coupling involves organelle- or adaptor-induced activation of the microtubule motors by conformational reversal of an autoinhibited state. Herein, we show that a similar regulatory mechanism operates for an adaptor protein named SKIP (also known as PLEKHM2). SKIP binds to the small guanosine triphosphatase (GTPase) ARL8 on the lysosomal membrane to couple lysosomes to the anterograde microtubule motor kinesin-1. Structure-function analyses of SKIP reveal that the C-terminal region comprising three pleckstrin homology (PH) domains interacts with the N-terminal region comprising ARL8- and kinesin-1-binding sites. This interaction inhibits coupling of lysosomes to kinesin-1 and, consequently, lysosome movement toward the cell periphery. We also find that ARL8 does not just recruit SKIP to the lysosomal membrane but also relieves SKIP autoinhibition, promoting kinesin-1-driven, anterograde lysosome transport. Finally, our analyses show that the largely disordered middle region of SKIP mediates self-association and that this self-association enhances the interaction of SKIP with kinesin-1. These findings indicate that SKIP is not just a passive connector of lysosome-bound ARL8 to kinesin-1 but is itself subject to intra- and inter-molecular interactions that regulate its function. We anticipate that similar organelle- or GTPase-induced conformational changes could regulate the activity of other kinesin adaptors.

Phagolysosome resolution requires contacts with the endoplasmic reticulum and phosphatidylinositol-4-phosphate signalling.

Phosphoinositides have a pivotal role in the maturation of nascent phagosomes into microbicidal phagolysosomes. Following degradation of their contents, mature phagolysosomes undergo resolution, a process that remains largely uninvestigated. Here we studied the role of phosphoinositides in phagolysosome resolution. Phosphatidylinositol-4-phosphate (PtdIns(4)P), which is abundant in maturing phagolysosomes, was depleted as they tubulated and resorbed. Depletion was caused, in part, by transfer of phagolysosomal PtdIns(4)P to the endoplasmic reticulum, a process mediated by oxysterol-binding protein-related protein 1L (ORP1L), a RAB7 effector. ORP1L formed discrete tethers between the phagolysosome and the endoplasmic reticulum, resulting in distinct regions with alternating PtdIns(4)P depletion and enrichment. Tubules emerged from PtdIns(4)P-rich regions, where ADP-ribosylation factor-like protein 8B (ARL8B) and SifA- and kinesin-interacting protein/pleckstrin homology domain-containing family M member 2 (SKIP/PLEKHM2) accumulated. SKIP binds preferentially to monophosphorylated phosphoinositides, of which PtdIns(4)P is most abundant in phagolysosomes, contributing to their tubulation. Accordingly, premature hydrolysis of PtdIns(4)P impaired SKIP recruitment and phagosome resolution. Thus, resolution involves phosphoinositides and tethering of phagolysosomes to the endoplasmic reticulum.

E. coli-Based Selection and Expression Systems for Discovery, Characterization, and Purification of Ubiquitylated Proteins.

Ubiquitylation is an eukaryotic signal that regulates most cellular pathways. However, four major hurdles pose challenges to study ubiquitylation: (1) high redundancy between ubiquitin (Ub) cascades, (2) ubiquitylation is tightly regulated in the cell, (3) the transient nature of the Ub signal, and (4) difficulties to purify functional ubiquitylation apparatus for in vitro assay. Here, we present systems that express functional Ub cascades in E. coli, which lacks deubiquitylases, Ub-dependent degradations, and control mechanisms for ubiquitylation. Therefore, expression of an ubiquitylation cascade results in the accumulation of stable ubiquitylated protein that can be genetically selected or purified, thus circumventing the above challenges. Co-expression of split antibiotic resistance protein fragments tethered to Ub and ubiquitylation targets along with ubiquitylation enzymes (E1, E2, and E3) gives rise to bacterial growth on selective media. We show that ubiquitylation rate is highly correlated with growth efficiency. Hence, genetic libraries and simple manipulations in the selection system facilitate the identification and characterization of components and interfaces along Ub cascades. The bacterial expression system also facilitates the detection of ubiquitylated proteins. Furthermore, the expression system allows affinity chromatography-based purification of milligram quantities of ubiquitylated proteins for downstream biochemical, biophysical, and structural studies.

A Ragulator-BORC interaction controls lysosome positioning in response to amino acid availability.

Lysosomes play key roles in the cellular response to amino acid availability. Depletion of amino acids from the medium turns off a signaling pathway involving the Ragulator complex and the Rag guanosine triphosphatases (GTPases), causing release of the inactive mammalian target of rapamycin complex 1 (mTORC1) serine/threonine kinase from the lysosomal membrane. Decreased phosphorylation of mTORC1 substrates inhibits protein synthesis while activating autophagy. Amino acid depletion also causes clustering of lysosomes in the juxtanuclear area of the cell, but the mechanisms responsible for this phenomenon are poorly understood. Herein we show that Ragulator directly interacts with BLOC-1-related complex (BORC), a multi-subunit complex previously found to promote lysosome dispersal through coupling to the small GTPase Arl8 and the kinesins KIF1B and KIF5B. Interaction with Ragulator exerts a negative regulatory effect on BORC that is independent of mTORC1 activity. Amino acid depletion strengthens this interaction, explaining the redistribution of lysosomes to the juxtanuclear area. These findings thus demonstrate that amino acid availability controls lysosome positioning through Ragulator-dependent, but mTORC1-independent, modulation of BORC.

Ubiquitylation-dependent oligomerization regulates activity of Nedd4 ligases.

Ubiquitylation controls protein function and degradation. Therefore, ubiquitin ligases need to be tightly controlled. We discovered an evolutionarily conserved allosteric restraint mechanism for Nedd4 ligases and demonstrated its function with diverse substrates: the yeast soluble proteins Rpn10 and Rvs167, and the human receptor tyrosine kinase FGFR1 and cardiac IKS potassium channel. We found that a potential trimerization interface is structurally blocked by the HECT domain α1-helix, which further undergoes ubiquitylation on a conserved lysine residue. Genetic, bioinformatics, biochemical and biophysical data show that attraction between this α1-conjugated ubiquitin and the HECT ubiquitin-binding patch pulls the α1-helix out of the interface, thereby promoting trimerization. Strikingly, trimerization renders the ligase inactive. Arginine substitution of the ubiquitylated lysine impairs this inactivation mechanism and results in unrestrained FGFR1 ubiquitylation in cells. Similarly, electrophysiological data and TIRF microscopy show that NEDD4 unrestrained mutant constitutively downregulates the IKS channel, thus confirming the functional importance of E3-ligase autoinhibition.

Mechanisms and functions of lysosome positioning.

Lysosomes have been classically considered terminal degradative organelles, but in recent years they have been found to participate in many other cellular processes, including killing of intracellular pathogens, antigen presentation, plasma membrane repair, cell adhesion and migration, tumor invasion and metastasis, apoptotic cell death, metabolic signaling and gene regulation. In addition, lysosome dysfunction has been shown to underlie not only rare lysosome storage disorders but also more common diseases, such as cancer and neurodegeneration. The involvement of lysosomes in most of these processes is now known to depend on the ability of lysosomes to move throughout the cytoplasm. Here, we review recent findings on the mechanisms that mediate the motility and positioning of lysosomes, and the importance of lysosome dynamics for cell physiology and pathology.

A bacterial genetic selection system for ubiquitylation cascade discovery.

About one-third of the eukaryotic proteome undergoes ubiquitylation, but the enzymatic cascades leading to substrate modification are largely unknown. We present a genetic selection tool that utilizes Escherichia coli, which lack deubiquitylases, to identify interactions along ubiquitylation cascades. Coexpression of split antibiotic resistance protein tethered to ubiquitin and ubiquitylation target together with a functional ubiquitylation apparatus results in a covalent assembly of the resistance protein, giving rise to bacterial growth on selective media. We applied the selection system to uncover an E3 ligase from the pathogenic bacteria EHEC and to identify the epsin ENTH domain as an ultraweak ubiquitin-binding domain. The latter was complemented with a structure-function analysis of the ENTH-ubiquitin interface. We also constructed and screened a yeast fusion library, discovering Sem1 as a novel ubiquitylation substrate of Rsp5 E3 ligase. Collectively, our selection system provides a robust high-throughput approach for genetic studies of ubiquitylation cascades and for small-molecule modulator screening.

Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism.

Ubiquitin receptors decode ubiquitin signals into many cellular responses. Ubiquitin receptors also undergo coupled monoubiquitylation, and rapid deubiquitylation has hampered the characterization of the ubiquitylated state. Using bacteria that express a ubiquitylation apparatus, we purified and determined the crystal structure of the proteasomal ubiquitin-receptor Rpn10 in its ubiquitylated state. The structure shows a novel ubiquitin-binding patch that directs K84 ubiquitylation. Superimposition of ubiquitylated-Rpn10 onto electron-microscopy models of proteasomes indicates that the Rpn10-conjugated ubiquitin clashes with Rpn9, suggesting that ubiquitylation might be involved in releasing Rpn10 from the proteasome. Indeed, ubiquitylation on immobilized proteasomes dissociates the modified Rpn10 from the complex, while unmodified Rpn10 mainly remains associated. In vivo experiments indicate that contrary to wild type, Rpn10-K84R is stably associated with the proteasomal subunit Rpn9. Similarly Rpn10, but not ubiquitylated-Rpn10, binds Rpn9 in vitro. Thus we suggest that ubiquitylation functions to dissociate modified ubiquitin receptors from their targets, a function that promotes cyclic activity of ubiquitin receptors.

Structure-based in silico identification of ubiquitin-binding domains provides insights into the ALIX-V:ubiquitin complex and retrovirus budding.

The ubiquitylation signal promotes trafficking of endogenous and retroviral transmembrane proteins. The signal is decoded by a large set of ubiquitin (Ub) receptors that tether Ub-binding domains (UBDs) to the trafficking machinery. We developed a structure-based procedure to scan the protein data bank for hidden UBDs. The screen retrieved many of the known UBDs. Intriguingly, new potential UBDs were identified, including the ALIX-V domain. Pull-down, cross-linking and E3-independent ubiquitylation assays biochemically corroborated the in silico findings. Guided by the output model, we designed mutations at the postulated ALIX-V:Ub interface. Biophysical affinity measurements using microscale-thermophoresis of wild-type and mutant proteins revealed some of the interacting residues of the complex. ALIX-V binds mono-Ub with a K(d) of 119 µM. We show that ALIX-V oligomerizes with a Hill coefficient of 5.4 and IC(50) of 27.6 µM and that mono-Ub induces ALIX-V oligomerization. Moreover, we show that ALIX-V preferentially binds K63 di-Ub compared with mono-Ub and K48 di-Ub. Finally, an in vivo functionality assay demonstrates the significance of ALIX-V:Ub interaction in equine infectious anaemia virus budding. These results not only validate the new procedure, but also demonstrate that ALIX-V directly interacts with Ub in vivo and that this interaction can influence retroviral budding.

Purification and crystallization of mono-ubiquitylated ubiquitin receptor Rpn10.

Protein ubiquitylation controls nearly all cellular pathways in eukaryotes. A repertoire of proteins named ubiquitin (Ub) receptors harbouring ubiquitin-binding domains (UBDs) recognize ubiquitylated proteins. These Ub receptors decode the Ub signal by tethering a UBD or UBDs to a functional domain or domains, thus linking the ubiquitylated target to a specific function. The rapid dynamics of ubiquitylation/deubiquitylation has impeded the characterization of ubiquitylated proteins. To bypass this obstacle, a recently developed synthetic system that reconstructs the entire eukaryotic ubiquitylation cascade in Escherichia coli was used to purify the mono-ubiquitylated form of the regulatory proteasomal non-ATPase subunit (Ub-Rpn10) from Saccharomyces cerevisiae. Here, the first crystallization and data collection of Ub-Rpn10 is reported. Purified Ub-Rpn10 was crystallized in 12%(w/v) PEG 20,000, 0.1 M MES pH 6.5 and yielded thin rhombus-shaped crystals. X-ray analysis revealed that these crystals belonged to the monoclinic system C2, with unit-cell parameters a = 107.3, b = 49.7, c = 81.3 Å, α = γ = 90.0, ß = 130.5°. A full synchrotron data set has been collected, merged and scaled with a diffraction limit of 3.14 Å.

Synthetic biology approach to reconstituting the ubiquitylation cascade in bacteria.

Covalent modification of proteins with ubiquitin (Ub) is widely implicated in the control of protein function and fate. Over 100 deubiquitylating enzymes rapidly reverse this modification, posing challenges to the biochemical and biophysical characterization of ubiquitylated proteins. We circumvented this limitation with a synthetic biology approach of reconstructing the entire eukaryotic Ub cascade in bacteria. Co-expression of affinity-tagged substrates and Ub with E1, E2 and E3 enzymes allows efficient purification of ubiquitylated proteins in milligram quantity. Contrary to in-vitro assays that lead to spurious modification of several lysine residues of Rpn10 (regulatory proteasomal non-ATPase subunit), the reconstituted system faithfully recapitulates its monoubiquitylation on lysine 84 that is observed in vivo. Mass spectrometry revealed the ubiquitylation sites on the Mind bomb E3 ligase and the Ub receptors Rpn10 and Vps9. Förster resonance energy transfer (FRET) analyses of ubiquitylated Vps9 purified from bacteria revealed that although ubiquitylation occurs on the Vps9-GEF domain, it does not affect the guanine nucleotide exchanging factor (GEF) activity in vitro. Finally, we demonstrated that ubiquitylated Vps9 assumes a closed structure, which blocks additional Ub binding. Characterization of several ubiquitylated proteins demonstrated the integrity, specificity and fidelity of the system, and revealed new biological findings.