A time-resolved Förster resonance energy transfer assay to investigate drug and inhibitor binding to ABCG2.

The human ATP-binding cassette (ABC) transporter, ABCG2, is responsible for multidrug resistance in some tumours. Detailed knowledge of its activity is crucial for understanding drug transport and resistance in cancer, and has implications for wider pharmacokinetics. The binding of substrates and inhibitors is a key stage in the transport cycle of ABCG2. Here, we describe a novel binding assay using a high affinity fluorescent inhibitor based on Ko143 and time-resolved Förster resonance energy transfer (TR-FRET) to measure saturation binding to ABCG2. This binding is displaced by Ko143 and other known ABCG2 ligands, and is sensitive to the addition of AMP-PNP, a non-hydrolysable ATP analogue. This assay complements the arsenal of methods for determining drug:ABCG2 interactions and has the possibility of being adaptable for other multidrug pumps.

Novel RAB27A Variant Associated with Late-Onset Hemophagocytic Lymphohistiocytosis Alters Effector Protein Binding.

Autosomal recessive mutations in RAB27A are associated with Griscelli syndrome type 2 (GS2), characterized by hypopigmentation and development of early-onset, potentially fatal hemophagocytic lymphohistiocytosis (HLH). We describe a 35-year old male who presented with recurrent fever, was diagnosed with Epstein-Barr virus-driven chronic lymphoproliferation, fulfilled clinical HLH criteria, and who carried a novel homozygous RAB27A c.551G > A p.(R184Q) variant. We aimed to evaluate the contribution of the identified RAB27A variant in regard to the clinical phenotype as well as cellular and biochemical function. The patient displayed normal pigmentation as well as RAB27A expression in blood-derived cells. However, patient NK and CD8+ T cell exocytosis was low. Ectopic expression of the RAB27A p.R184Q variant rescued melanosome distribution in mouse Rab27a-deficient melanocytes, but failed to increase exocytosis upon reconstitution of human RAB27A-deficient CD8+ T cells. Mechanistically, the RAB27A p.R184Q variant displayed reduced binding to SLP2A but augmented binding to MUNC13-4, two key effector proteins in immune cells. MUNC13-4 binding was particularly strong to an inactive RAB27A p.T23N/p.R184Q double mutant. RAB27A p.R184Q was expressed and could facilitate melanosome trafficking, but did not support lymphocyte exocytosis. The HLH-associated RAB27A variant increased Munc13-4 binding, potentially representing a novel mode of impairing RAB27A function selectively in hematopoietic cells.

Nucleotide exchange factor Rab3GEP requires DENN and non-DENN elements for activation and targeting of Rab27a.

Rab GTPases are compartment-specific molecular switches that regulate intracellular vesicular transport in eukaryotes. GDP/GTP exchange factors (GEFs) control Rab activation, and current models propose that localised and regulated GEF activity is important in targeting Rabs to specific membranes. Here, we investigated the mechanism of GEF function using the Rab27a GEF, Rab3GEP (also known as MADD), in melanocytes as a model. We show that Rab3GEP-deficient melanocytes (melan-R3GKO) manifest partial disruption of melanosome dispersion, a read-out of Rab27a activation and targeting. Using rescue of melanosome dispersion in melan-R3GKO cells and effector pull-down approaches we show that the DENN domain of Rab3GEP (conserved among RabGEFs) is necessary, but insufficient, for its cellular function and GEF activity. Finally, using a mitochondrial re-targeting strategy, we show that Rab3GEP can target Rab27a to specific membranes in a GEF-dependent manner. We conclude that Rab3GEP facilitates the activation and targeting of Rab27a to specific membranes, but that it differs from other DENN-containing RabGEFs in requiring DENN and non-DENN elements for both of these activities and by lacking compartment-specific localisation.

Cross-linking, DEER-spectroscopy and molecular dynamics confirm the inward facing state of P-glycoprotein in a lipid membrane.

The drug efflux pump P-glycoprotein (P-gp) displays a complex transport mechanism involving multiple drug binding sites and two centres for nucleotide hydrolysis. Elucidating the molecular mechanism of transport remains elusive and the availability of P-gp structures in distinct natural and ligand trapped conformations will accelerate our understanding. The present investigation sought to provide biochemical data to validate specific features of these structures; with particular focus on the transmembrane domain that provides the transport conduit. Hence our focus was on transmembrane helices six and twelve (TM6/TM12), which are believed to participate in drug binding, as they line the central transport conduit and provide a direct link to the catalytic centres. A series of P-gp mutants were generated with a single cysteine in both TM6 and TM12 to facilitate measurement of inter-helical distances using cross-linking and DEER strategies. Experimental results were compared to published structures per se and those refined by MD simulations. This analysis revealed that the refined inward-facing murine structure (4M1M) of P-gp provides a good representation of the proximity, topography and relative motions of TM6 and TM12 in reconstituted human P-gp.

Disruption of the Unique ABCG-Family NBD:NBD Interface Impacts Both Drug Transport and ATP Hydrolysis.

ABCG2 is one of a triumvirate of human multidrug ATP binding cassette (ABC) transporters that are implicated in the defense of cells and tissues against cytotoxic chemicals, but these transporters can also confer chemotherapy resistance states in oncology. Understanding the mechanism of ABCG2 is thus imperative if we are to be able to counter its deleterious activity. The structure of ABCG2 and its related family members (ABCG5/G8) demonstrated that there were two interfaces between the nucleotide binding domains (NBD). In addition to the canonical ATP "sandwich-dimer" interface, there was a second contact region between residues at the C-terminus of the NBD. We investigated this second interface by making mutations to a series of residues that are in close interaction with the opposite NBD. Mutated ABCG2 isoforms were expressed in human embryonic kidney (HEK) 293T cells and analysed for targeting to the membrane, drug transport, and ATPase activity. Mutations to this second interface had a number of effects on ABCG2, including altered drug specificity, altered drug transport, and, in two mutants, a loss of ATPase activity. The results demonstrate that this region is particularly sensitive to mutation and can impact not only direct, local NBD events (i.e., ATP hydrolysis) but also the allosteric communication to the transmembrane domains and drug transport.

Inefficient recruitment of kinesin-1 to melanosomes precludes it from facilitating their transport.

Microtubules and F-actin, and their associated motor proteins, are considered to play complementary roles in long- and short-range organelle transport. However, there is growing appreciation that myosin/F-actin networks can drive long-range transport. In melanocytes, myosin-Va and kinesin-1 have both been proposed as long-range centrifugal transporters moving melanosomes into the peripheral dendrites. Here, we investigated the role of kinesin-1 heavy chain (Kif5b) and its suggested targeting factor Rab1a in transport. We performed confocal microscopy and subcellular fractionation, but did not detect Kif5b or Rab1a on melanosomes. Meanwhile functional studies, using siRNA knockdown and dominant negative mutants, did not support a role for Kif5b or Rab1a in melanosome transport. To probe the potential of Kif5b to function in transport, we generated fusion proteins that target active Kif5b to melanosomes and tested their ability to rescue perinuclear clustering in myosin-Va-deficient cells. Expression of these chimeras, but not full-length Kif5b, dispersed melanosomes with similar efficiency to myosin-Va. Our data indicate that kinesin and microtubules can compensate for defects in myosin-Va and actin-based transport in mammals, but that endogenous Kif5b does not have an important role in transport of melanocytes due to its inefficient recruitment to melanosomes.

Residues contributing to drug transport by ABCG2 are localised to multiple drug-binding pockets.

Multidrug binding and transport by the ATP-binding cassette transporter ABCG2 is a factor in the clinical resistance to chemotherapy in leukaemia, and a contributory factor to the pharmacokinetic profiles of many other prescribed drugs. Despite its importance, the structural basis of multidrug transport, i.e. the ability to transport multiple distinct chemicals, has remained elusive. Previous research has shown that at least two residues positioned towards the cytoplasmic end of transmembrane helix 3 (TM3) of the transporter play a role in drug transport. We hypothesised that other residues, either in the longitudinal span of TM3, or a perpendicular slice through the intracellular end of other TM helices would also contribute to drug binding and transport by ABCG2. Single-point mutant isoforms of ABCG2 were made at ∼30 positions and were analysed for effects on protein expression, localisation (western blotting, confocal microscopy) and function (flow cytometry) in a mammalian stable cell line expression system. Our data were interpreted in terms of recent structural data on the ABCG protein subfamily and enabled us to propose a surface-binding site for the drug mitoxantrone (MX) as well as a second, buried site for the same drug. Further mutational analysis of residues that spatially separate these two sites prompts us to suggest a molecular and structural pathway for MX transport by ABCG2.

Actomyosin organelle functions of SPIRE actin nucleators precede animal evolution.

An important question in cell biology is how cytoskeletal proteins evolved and drove the development of novel structures and functions. Here we address the origin of SPIRE actin nucleators. Mammalian SPIREs work with RAB GTPases, formin (FMN)-subgroup actin assembly proteins and class-5 myosin (MYO5) motors to transport organelles along actin filaments towards the cell membrane. However, the origin and extent of functional conservation of SPIRE among species is unknown. Our sequence searches show that SPIRE exist throughout holozoans (animals and their closest single-celled relatives), but not other eukaryotes. SPIRE from unicellular holozoans (choanoflagellate), interacts with RAB, FMN and MYO5 proteins, nucleates actin filaments and complements mammalian SPIRE function in organelle transport. Meanwhile SPIRE and MYO5 proteins colocalise to organelles in Salpingoeca rosetta choanoflagellates. Based on these observations we propose that SPIRE originated in unicellular ancestors of animals providing an actin-myosin driven exocytic transport mechanism that may have contributed to the evolution of complex multicellular animals.

Vinca alkaloid binding to P-glycoprotein occurs in a processive manner.

A mechanistic understanding of how P-glycoprotein (Pgp) is able to bind and transport its astonishing range of substrates remains elusive. Pharmacological data demonstrated the presence of at least four distinct binding sites, but their locations have not been fully elucidated. The combination of biochemical and structural data suggests that initial binding may occur in the central cavity or at the lipid-protein interface. Our objective was to define the binding sites for two transported substrates of Pgp; the anticancer drug vinblastine and the fluorescent probe rhodamine 123. A series of mutations was generated in positions proximal to previously defined drug-interacting residues on Pgp. The protein was purified and reconstituted into styrene-maleic acid lipid particles (SMALPs) to measure the apparent drug binding constant or into liposomes for assessment of drug-stimulated ATP hydrolysis. The biochemical data were reconciled with structural models of Pgp using molecular docking. The data indicated that the binding of rhodamine 123 occurred predominantly within the central cavity of Pgp. In contrast, the significantly more hydrophobic vinblastine bound to both the lipid-protein interface and within the central cavity. The data suggest that the initial interaction of vinca alkaloids with Pgp occurs at the lipid interface followed by internalisation into the central cavity, which also provides the transport conduit. This model is supported by recent structural observations with Pgp and early biophysical and cross-linking approaches. Moreover, the proposed model illustrates that the broad substrate profile for Pgp is underpinned by a combination of multiple initial interaction sites and an accommodating transport conduit.

Application of fluorescence correlation spectroscopy to study substrate binding in styrene maleic acid lipid copolymer encapsulated ABCG2.

ABCG2 is one of a trio of human ATP binding cassette transporters that have the ability to bind and transport a diverse array of chemical substrates out of cells. This so-called "multidrug" transport has numerous physiological consequences including effects on how drugs are absorbed into and eliminated from the body. Understanding how ABCG2 is able to interact with multiple drug substrates remains an important goal in transporter biology. Most drugs are believed to interact with ABCG2 through the hydrophobic lipid bilayer and experimental systems for ABCG2 study need to incorporate this. We have exploited styrene maleic acid to solubilise ABCG2 from HEK293T cells overexpressing the transporter, and confirmed by dynamic light scattering and fluorescence correlation spectroscopy (FCS) that this results in the extraction of SMA lipid copolymer (SMALP) particles that are uniform in size and contain a dimer of ABCG2, which is the predominant physiological state. FCS was further employed to measure the diffusion of a fluorescent ABCG2 substrate (BODIPY-prazosin) in the presence and absence of SMALP particles of purified ABCG2. Autocorrelation analysis of FCS traces enabled the mathematical separation of free BODIPY-prazosin from drug bound to ABCG2 and allowed us to show that combining SMALP extraction with FCS can be used to study specific drug: transporter interactions.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins.

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

The adaptor protein melanophilin regulates dynamic myosin-Va:cargo interaction and dendrite development in melanocytes.

The regulation of organelle transport by the cytoskeleton is fundamental for eukaryotic survival. Cytoskeleton motors are typically modular proteins with conserved motor and diverse cargo-binding domains. Motor:cargo interactions are often indirect and mediated by adaptor proteins, for example, Rab GTPases. Rab27a, via effector melanophilin (Mlph), recruits myosin-Va (MyoVa) to melanosomes and thereby disperses them into melanocyte dendrites. To better understand how adaptors regulate motor:cargo interaction, we used single melanosome fluorescence recovery after photobleaching (smFRAP) to characterize the association kinetics among MyoVa, its adaptors, and melanosomes. We found that MyoVa and Mlph rapidly recovered after smFRAP, whereas Rab27a did not, indicating that MyoVa and Mlph dynamically associate with melanosomes and Rab27a does not. This suggests that dynamic Rab27a:effector interaction rather than Rab27a melanosome:cytosol cycling regulates MyoVa:melanosome association. Accordingly, a Mlph-Rab27a fusion protein reduced MyoVa smFRAP, indicating that it stabilized melanosomal MyoVa. Finally, we tested the functional importance of dynamic MyoVa:melanosome interaction. We found that whereas a MyoVa-Rab27a fusion protein dispersed melanosomes in MyoVa-deficient cells, dendrites were significantly less elongated than in wild-type cells. Given that dendrites are the prime sites of melanosome transfer from melanocytes to keratinocytes, we suggest that dynamic MyoVa:melanosome interaction is important for pigmentation in vivo.

Myosin-Va and dynamic actin oppose microtubules to drive long-range organelle transport.

In animal cells, microtubule and actin tracks and their associated motors (dynein, kinesin, and myosin) are thought to regulate long- and short-range transport, respectively. Consistent with this, microtubules extend from the perinuclear centrosome to the plasma membrane and allow bidirectional cargo transport over long distances (>1 µm). In contrast, actin often comprises a complex network of short randomly oriented filaments, suggesting that myosin motors move cargo short distances. These observations underpin the "highways and local roads" model for transport along microtubule and actin tracks. The "cooperative capture" model exemplifies this view and suggests that melanosome distribution in melanocyte dendrites is maintained by long-range transport on microtubules followed by actin/myosin-Va-dependent tethering. In this study, we used cell normalization technology to quantitatively examine the contribution of microtubules and actin/myosin-Va to organelle distribution in melanocytes. Surprisingly, our results indicate that microtubules are essential for centripetal, but not centrifugal, transport. Instead, we find that microtubules retard a centrifugal transport process that is dependent on myosin-Va and a population of dynamic F-actin. Functional analysis of mutant proteins indicates that myosin-Va works as a transporter dispersing melanosomes along actin tracks whose +/barbed ends are oriented toward the plasma membrane. Overall, our data highlight the role of myosin-Va and actin in transport, and not tethering, and suggest a new model in which organelle distribution is determined by the balance between microtubule-dependent centripetal and myosin-Va/actin-dependent centrifugal transport. These observations appear to be consistent with evidence coming from other systems showing that actin/myosin networks can drive long-distance organelle transport and positioning.

Induction of a stress response in Lactococcus lactis is associated with a resistance to ribosomally active antibiotics.

The acquisition of multidrug resistance in bacteria underlies the failure of antimicrobial therapy, and the emergence of pathogens that are resistant to almost the entire armoury of antibiotics. Among the proteins that can mediate or contribute to the drug-resistance profile in Gram-positive bacteria is a subset of ATP-binding cassette proteins that are comprised of a tandem-repeated nucleotide-binding domain. In this study, we expressed one of these NBD(2) proteins, LmrC, in an antibiotic-sensitive Gram-positive host strain (Lactococcus lactis) and demonstrated the acquisition of resistance to ribosomally active antibiotics. Mutation of key catalytic residues suggested that the resistance profile was the result of a cellular response, rather than being a function of the NBD(2) protein itself. This observation was confirmed by 2D SDS/PAGE, which demonstrated that the expression of the NBD(2) protein induced a stress response in L. lactis. A model combining this stress response induction and the acquisition of antibiotic resistance is proposed.