Biochemical characterization of transporter associated with antigen processing (TAP)-like (ABCB9) expressed in insect cells

Identification of two novel heterodimeric ABC transporters in melanoma: ABCB5β/B6 and ABCB5β/B9

ABCB5 is a member of the ABC transporter superfamily composed of 48 transporters, which have been extensively studied for their role in cancer multidrug resistance and, more recently, in tumorigenesis. ABCB5 has been identified as a marker of skin progenitor cells, melanoma, and limbal stem cells. It has also been associated with multidrug resistance in several cancers. The unique feature of ABCB5 is that it exists as both a full transporter (ABCB5FL) and a half transporter (ABCB5β). Several studies have shown that the ABCB5β homodimer does not confer multidrug resistance, in contrast to ABCB5FL. In this study, using three complementary techniques, (1) nanoluciferase-based bioluminescence resonance energy transfer, (2) coimmunoprecipitation, and (3) proximity ligation assay, we identified two novel heterodimers in melanoma: ABCB5β/B6 and ABCB5β/B9. Both heterodimers could be expressed in High-Five insect cells and ATPase assays revealed that both functional nucleotide-binding domains of homodimers and heterodimers are required for their basal ATPase activity. These results are an important step toward elucidating the functional role of ABCB5β in melanocytes and melanoma.

Nucleotide dependence of the dimerization of ATP binding cassette nucleotide binding domains

ATP-binding cassette proteins are ubiquitously present throughout all known genomes. Their basic functional unit possesses two transmembrane domains and two nucleotide-binding domains. The nucleotide-binding domains are responsible for ATP binding and hydrolysis, and their 3-dimensional structure is conserved across ATP-binding cassette proteins. Binding of ATP produces nucleotide-binding domain dimerization, a step necessary for hydrolysis. However, the possibility that nucleotide-binding domains bind and/or hydrolyze nucleotide triphosphates different from ATP has not been explored in detail. Here, we studied that possibility using M. jannaschii MJ0796, a prototypical ATP-binding cassette nucleotide-binding domain. We found that nucleotide-binding domain dimerization occurs as a result of binding to the natural nucleotide triphosphates ATP, GTP, CTP and UTP, and also to the analog ATP-γ-S. All the natural nucleotide triphosphates are hydrolyzed at similar rates, whereas ATP-γ-S is not hydrolyzed. We also found that the non-hydrolyzable ATP analog AMP-PNP, frequently assumed to produce the nucleotide-bound conformation, failed to elicit nucleotide-binding domain dimerization. Our results raise the possibility that not all the nucleotide binding sites of nucleotide-binding domains are occupied by ATP under physiological conditions, and that ATP is not always the nucleotide hydrolyzed to dissociate the nucleotide-binding domain dimers.

Co-operative function and mutual stabilization of the half ATP-binding cassette transporters HAF-4 and HAF-9 in Caenorhabditis elegans

Caenorhabditis elegans HAF-4 and HAF-9 are half ABC (ATP-binding-cassette) transporters that are highly homologous to the human lysosomal peptide transporter TAPL [TAP (transporter associated with antigen processing)-like; ABCB9]. We reported previously that both HAF-4 and HAF-9 localize to the membrane of a subset of intestinal organelles, and are required for the formation of these organelles and other physiological aspects. In the present paper, we report the genetic and physical interactions between HAF-4 and HAF-9. Overexpression of HAF-4 and HAF-9 did not rescue the intestinal organelle defect of the haf-9 and haf-4 deletion mutants respectively, indicating that they cannot substitute for each other. Double haf-4 and haf-9 mutants do not exhibit more severe phenotypes than the single mutants, suggesting their co-operative function. Immunoprecipitation experiments demonstrated their physical interaction. The results of the present study suggest that HAF-4 and HAF-9 form a heterodimer. Furthermore, Western blot analysis of the deletion mutants and RNAi (RNA interference) knockdown experiments in GFP (green fluorescent protein)-tagged HAF-4 or HAF-9 transgenic worms suggest that HAF-4-HAF-9 heterodimer formation is required for their stabilization. The findings provide a clue as to how ABC transporters adopt a stable functional form.

The lysosomal polypeptide transporter TAPL: more than a housekeeping factor?

The transporter associated with antigen processing-like (TAPL) is a polypeptide transporter translocating cytosolic peptides into the lumen of lysosomes driven by ATP hydrolysis. TAPL belongs to the family of ABC transporters and forms a homodimer. This ABC transporter not only shows a broad tissue but also a wide phylogenetic distribution, because orthologs are still found in nematodes and insects. Here, we present the topology, substrate specificity, and distribution of this intracellular polypeptide transporter. Additionally, we will discuss its proposed physiological functions such as housekeeping together with a specialized factor for metabolite storage as well as for the adaptive immunity.

Expression of membrane proteins in Drosophila Melanogaster S2 cells: Production and analysis of a EGFP-fused G protein-coupled receptor as a model

In the process of selecting an appropriate host for the heterologous expression of functional eukaryotic membrane proteins, Drosophila S2 cells, although not yet fully explored, appear as a valuable alternative to mammalian cell lines or other virus-infected insect cell systems. This nonlytic, plasmid-based system actually combines several major physiological and bioprocess advantages that make it a highly potential and scalable cellular tool for the production of membrane proteins in a variety of applications, including functional characterization, pharmacological profiling, molecular simulations, structural analyses, or generation of vaccines. We present here a series of protocols and hints that would serve the successful expression of membrane proteins in S2 cells, using an enhanced green fluorescent protein (EGFP)/G protein-coupled receptor (EGFP-GPCR) as a model.

Oligomerization of human ATP-binding cassette transporters and its potential significance in human disease

Human ATP-binding cassette transporters (ABC transporter) belong to an extremely important superfamily of membrane transporters. They use energy from ATP hydrolysis to transport a wide variety of substrates across the cellular membrane. Due to the physiological and pharmacological importance of their diverse substrates, ABC transporters have been shown to have close relationship with various human diseases such as cystic fibrosis and multi-drug resistance in cancer chemotherapy. While it has been thought traditionally that functional ABC transporters exist as monomeric full or dimeric half transporters, emerging evidence indicates that some ABC transporters oligomerize on cellular membranes and this oligomerization seems to have functional relevance. Therefore, this oligomerization process might be a promising drug target for ABC transporter-related human diseases, especially in overcoming multi-drug resistance in cancer chemotherapy. In this review, we perform a critical analysis of the past studies on the oligomerization of ABC transporters.

Normal formation of a subset of intestinal granules in Caenorhabditis elegans requires ATP-binding cassette transporters HAF-4 and HAF-9, which are highly homologous to human lysosomal peptide transporter TAP-like

TAP-like (TAPL; ABCB9) is a half-type ATP-binding cassette (ABC) transporter that localizes in lysosome and putatively conveys peptides from cytosol to lysosome. However, the physiological role of this transporter remains to be elucidated. Comparison of genome databases reveals that TAPL is conserved in various species from a simple model organism, Caenorhabditis elegans, to mammals. C. elegans possesses homologous TAPL genes: haf-4 and haf-9. In this study, we examined the tissue-specific expression of these two genes and analyzed the phenotypes of the loss-of-function mutants for haf-4 and haf-9 to elucidate the in vivo function of these genes. Both HAF-4 and HAF-9 tagged with green fluorescent protein (GFP) were mainly localized on the membrane of nonacidic but lysosome-associated membrane protein homologue (LMP-1)-positive intestinal granules from larval to adult stage. The mutants for haf-4 and haf-9 exhibited granular defects in late larval and young adult intestinal cells, associated with decreased brood size, prolonged defecation cycle, and slow growth. The intestinal granular phenotype was rescued by the overexpression of the GFP-tagged wild-type protein, but not by the ATP-unbound form of HAF-4. These results demonstrate that two ABC transporters, HAF-4 and HAF-9, are related to intestinal granular formation and some other physiological aspects.