Recombinant Doxorubicin resistance ABC transporter permease protein drrB (drrB)

What is DrrB and how does it function within the DrrAB complex?

DrrB is the integral membrane protein component of the bacterial DrrAB multidrug transporter, serving as the transmembrane domain (TMD) that forms the conduit for drug export. DrrB works in conjunction with DrrA, which functions as the nucleotide-binding domain (NBD) that hydrolyzes ATP. Together, they form a tetrameric DrrA₂B₂ complex that constitutes a functional ABC transporter. DrrB contains the drug-binding sites, while DrrA provides the energy through ATP hydrolysis to power the conformational changes required for drug translocation across the membrane. This system represents one of the simplest forms of multidrug transporters in the ABC superfamily, making it an excellent model for understanding the mechanisms of multidrug recognition and transport .

How is the drug-binding pocket of DrrB structured?

DrrB contains a large and flexible drug-binding pocket composed of aromatic residues contributed by several transmembrane helices. This pocket accommodates multiple drugs, with different compounds binding to both specific and shared residues within the pocket. The structural flexibility of this pocket allows DrrB to recognize and bind a diverse range of substrates with varying affinities. Fluorescence-based studies have demonstrated that some drugs exhibit multiple binding sites within DrrB, while others bind to a single site. This adaptable binding pocket is characteristic of multidrug transporters that must accommodate structurally diverse compounds .

What is the relationship between DrrB and doxorubicin resistance?

DrrB plays a critical role in doxorubicin resistance by facilitating the ATP-dependent efflux of doxorubicin (Dox) and daunorubicin (Dnr) out of cells. As part of the DrrAB complex, DrrB forms the transmembrane channel through which these drugs are exported, effectively reducing their intracellular concentration and diminishing their cytotoxic effects. The development of doxorubicin resistance is a significant clinical challenge in cancer treatment, particularly in soft-tissue sarcomas where doxorubicin is used as first-line therapy. Understanding the structure and function of DrrB provides insights into mechanisms of drug resistance and potential strategies to overcome it .

What are the kinetic parameters of drug binding to the DrrAB complex?

The DrrAB complex demonstrates variable affinity for different drugs, with distinct binding kinetics that provide insights into its substrate specificity. Fluorescence-based studies have revealed that many drugs exhibit two binding sites with dramatically different affinities. For example, doxorubicin binds to DrrAB with Kd values of 0.19 ± 0.02 μM for the high-affinity site and 112 ± 9.20 μM for the low-affinity site. Other drugs show similar patterns: Hoechst 33342 (0.03 ± 0.006 μM and 11 ± 0.40 μM), rifampicin (0.13 ± 0.02 μM and 36 ± 2.50 μM), and rhodamine B (0.08 ± 0.01 μM and 74 ± 4.00 μM). In contrast, ethidium bromide and verapamil appear to bind to a single site with Kd values of 26 ± 0.60 μM and 0.07 ± 0.01 μM, respectively. These diverse binding affinities reflect the complex nature of drug recognition by the DrrAB transporter .

How do conformational changes propagate between DrrA and DrrB during transport?

The transduction of conformational changes between DrrA and DrrB involves specific conserved motifs that serve as communication interfaces. The pathway includes the Q-loop and CREEM (C-terminal Regulatory Glu-Glu-Met) motifs in DrrA and the EAA-like motif in DrrB. In the resting state, the EAA-like motif of DrrB is in contact with the CREEM motif of DrrA. During drug transport, the EAA-like motif disengages from the CREEM motif and forms a contact with the Q-loop in the NBD of DrrA. This dynamic switching of interactions enables the transmission of conformational changes from the drug-binding events in DrrB to the ATP hydrolysis in DrrA, and vice versa, ultimately resulting in the alternation of TMD conformations necessary for drug export. Fluorescence labeling experiments have provided clear evidence of these long-range conformational changes between the two subunits .

What is the nature of nucleotide binding to the DrrAB complex?

The DrrAB complex exhibits asymmetric nucleotide binding characteristics with two distinct binding sites in DrrA that have strikingly different affinities. For ATP, the high-affinity site has a Kd of 33 ± 4.50 μM, while the low-affinity site has a Kd of 2011 ± 807 μM. Similar asymmetry is observed for ADP (22 ± 2.10 μM and 4847 ± 957 μM) and AMP (18 ± 1.59 μM and 4817 ± 998 μM). Interestingly, DrrAB shows approximately 100-fold higher affinity for GTP (Kd of 0.39 ± 0.03 μM for the high-affinity site) compared to ATP, suggesting that GTP may be the preferred energy source for this transporter in vitro. This functional asymmetry of nucleotide binding sites is reminiscent of that observed in P-glycoprotein and other ABC transporters, although the mechanism by which this asymmetry is created in DrrAB may involve specific interactions between the two DrrA subunits or their associations with DrrB .

How does the presence of ATP affect drug binding to DrrB?

ATP binding to DrrA induces conformational changes that alter the drug-binding properties of DrrB. Fluorescence studies have shown that the presence of ATP affects the binding affinity of drugs to DrrB. For example, the Kd values for doxorubicin change from 0.19 ± 0.02 μM and 112 ± 9.20 μM in the absence of ATP to 0.35 ± 0.05 μM and 241 ± 20.14 μM after ATP binding. Similarly, for vinblastine, the Kd values change from 0.26 ± 0.04 μM and 93 ± 12.00 μM to 0.45 ± 0.09 μM and 193 ± 27.67 μM after ATP binding. These changes indicate that ATP binding to DrrA induces conformational changes in DrrB that modify its drug-binding pocket, potentially preparing the transporter for the drug efflux process .

What fluorescence-based methods are effective for studying DrrB-drug interactions?

Fluorescence-based approaches have proven highly effective for investigating drug binding to DrrB and conformational changes in the DrrAB complex. Two primary methods include:

• Intrinsic tryptophan fluorescence: This approach utilizes the natural fluorescence of tryptophan residues in DrrB to monitor changes in their local environment upon drug binding. When drugs bind to DrrB, they can quench the tryptophan fluorescence, providing a direct measure of binding affinity and stoichiometry. This method has the advantage of analyzing unmodified proteins in their native state.

• Extrinsic cysteine-based fluorescent probes: This approach involves targeted labeling of specific cysteine residues with fluorescent probes like IAANS (2-(4′-(iodoacetamido)anilino)naphthalene-6-sulfonic acid). These probes serve as reporters of conformational changes in specific regions of the protein. This method allows for more precise localization of conformational changes resulting from drug or nucleotide binding.

Both approaches are highly sensitive to environmental changes surrounding the fluorescent probe and have been successfully employed to demonstrate high-affinity binding of multiple drugs and nucleotides to DrrAB, as well as to characterize the conformational changes between DrrA and DrrB during the transport cycle .

How can researchers effectively measure DrrB-mediated drug efflux?

To effectively measure DrrB-mediated drug efflux, researchers can employ several complementary approaches:

• Inside-out vesicle (IOV) assays: Prepare IOVs from cells expressing DrrAB and measure ATP-dependent accumulation of fluorescent drugs like doxorubicin or Hoechst 33342 inside the vesicles. This approach directly measures the transport activity of the DrrAB complex.

• Whole-cell efflux assays: Monitor the efflux of fluorescent substrates from cells expressing DrrAB using real-time fluorescence measurements. This approach provides information about transport kinetics in a cellular context.

• Competition assays: Assess the ability of non-fluorescent drugs to inhibit the efflux of a fluorescent reporter substrate. This approach can identify substrates that might not be directly measurable and can provide insights into binding site interactions.

• ATPase activity assays: Measure the drug-stimulated ATPase activity of the DrrAB complex as an indirect indicator of transport function. This approach links ATP hydrolysis to drug binding and transport.

These methods, when used in combination, provide a comprehensive assessment of DrrB-mediated drug transport and can reveal important insights into substrate specificity, transport kinetics, and the effects of mutations or inhibitors on transport function .

How can understanding DrrB function inform strategies to overcome doxorubicin resistance?

Understanding the structure and function of DrrB provides several avenues for developing strategies to overcome doxorubicin resistance:

• Inhibitor development: Knowledge of the drug-binding pocket of DrrB can guide the design of specific inhibitors that block drug efflux without interfering with the action of chemotherapeutic agents. Such inhibitors could potentially reverse doxorubicin resistance in cancer cells.

• Combination therapies: The synergistic effect observed between recombinant methioninase (rMETase) and doxorubicin against fibrosarcoma cells suggests that targeting alternative metabolic pathways (like methionine dependency) can enhance the efficacy of doxorubicin even in resistant cells. The IC50 for rMETase was found to be 0.42 U/ml for doxorubicin-resistant HT1080 (DR-HT1080) cells, indicating its effectiveness against doxorubicin-resistant cancer cells .

• Structural modifications: Understanding how doxorubicin interacts with DrrB can inform structural modifications to doxorubicin or the development of analogs that maintain cytotoxicity but are poor substrates for efflux transporters.

• Allosteric modulators: Targeting the conformational changes between DrrA and DrrB with molecules that stabilize or disrupt specific conformational states could potentially interfere with the drug transport cycle and overcome resistance.

These approaches, informed by detailed molecular understanding of DrrB structure and function, offer promising avenues for addressing the clinical challenge of doxorubicin resistance in cancer treatment .

What is the potential of rMETase in combination with doxorubicin for treating drug-resistant cancers?

Research has demonstrated significant potential for recombinant methioninase (rMETase) in combination with doxorubicin for treating drug-resistant cancers, particularly fibrosarcoma. Key findings include:

• Selective synergy: The combination of rMETase and doxorubicin shows synergistic effects specifically against fibrosarcoma cells (HT1080) but not against normal fibroblasts (Hs27). This selectivity is crucial for minimizing toxicity to normal tissues during cancer treatment.

• Efficacy against resistant cells: rMETase alone is highly effective against doxorubicin-resistant HT1080 (DR-HT1080) cells, with an IC50 of 0.42 U/ml compared to 0.75 U/ml for parental HT1080 cells. This suggests that methionine dependency remains a vulnerability even in doxorubicin-resistant cancer cells.

• Enhanced nuclear fragmentation: The combination of doxorubicin and rMETase causes more fragmented nuclei than either treatment alone in HT1080 cells, indicating increased cytotoxicity through potentially different mechanisms.

• Differential sensitivity: The IC50 for doxorubicin was 3.3 μM for HT1080 cells, 12.4 μM for DR-HT1080 cells (showing resistance), and 7.25 μM for Hs27 cells. The combination therapy effectively addresses this resistance.

These findings suggest that combining rMETase with doxorubicin represents a promising therapeutic strategy for treating fibrosarcoma, including doxorubicin-resistant cases. The approach leverages the cancer-specific vulnerability of methionine addiction alongside conventional chemotherapy to overcome drug resistance mechanisms .

What are the key unresolved questions about DrrB structure and function?

Despite significant advances in understanding DrrB, several key questions remain unresolved:

• High-resolution structure: The atomic-level structure of DrrB, particularly in complex with DrrA and in different conformational states, remains to be determined. Such structural information would provide crucial insights into the drug-binding pocket, the interface with DrrA, and the conformational changes during transport.

• Complete substrate profile: While several drugs have been shown to bind to DrrB, a comprehensive profile of all potential substrates and their binding modes would enhance our understanding of substrate recognition and selectivity.

• Conformational dynamics: The precise sequence and timing of conformational changes during the transport cycle, particularly how ATP binding and hydrolysis in DrrA coordinates with drug binding and release in DrrB, require further elucidation.

• Regulatory mechanisms: Potential regulatory mechanisms that modulate DrrB function, such as post-translational modifications or interactions with other cellular components, remain largely unexplored.

• Evolutionary relationships: A deeper understanding of how DrrB relates to other bacterial and eukaryotic drug transporters would provide insights into the evolution of multidrug resistance mechanisms.

Addressing these questions will require integrating multiple experimental approaches, including structural biology, biochemistry, biophysics, and computational modeling, to build a comprehensive understanding of DrrB function in the context of multidrug resistance .

How can the DrrAB system serve as a model for studying clinically relevant multidrug transporters?

The DrrAB system offers several advantages as a model for studying clinically relevant multidrug transporters:

• Structural simplicity: The separation of the NBD (DrrA) and TMD (DrrB) on distinct polypeptides simplifies the study of domain interactions and conformational coupling, providing a clearer picture of the fundamental mechanisms of ABC transporters.

• Functional homology: DrrAB exhibits overlapping substrate specificity with mammalian P-glycoprotein, suggesting conserved mechanisms of multidrug recognition and transport across evolutionary distant transporters.

• Experimental accessibility: The bacterial expression system for DrrAB allows for relatively straightforward genetic manipulation, protein expression, and purification compared to many eukaryotic transporters.

• Defined interaction interfaces: The well-characterized interfaces between DrrA and DrrB, including the Q-loop/EAA-like motif and CREEM/EAA-like motif interactions, provide specific targets for investigating the mechanisms of conformational coupling between NBDs and TMDs.

Insights gained from the DrrAB system can inform our understanding of more complex eukaryotic multidrug transporters implicated in clinical drug resistance, potentially leading to the development of more effective strategies to overcome multidrug resistance in human diseases .