Endomembrane-biased dimerization of ABCG16 and ABCG25 transporters determines their substrate selectivity in ABA-regulated plant growth and stress responses

ATP-binding cassette(ABC)transporters are integral membrane proteins that have evolved diverse func-tions fulfilled via the transport of various substrates.In Arabidopsis,the G subfamily of ABC proteins is particularly abundant and participates in multiple signaling pathways during plant development and stress responses.In this study,we revealed that two Arabidopsis ABCG transporters,ABCG16 and ABCG25,engage in ABA-mediated stress responses and early plant growth through endomembrane-specific dimerization-coupled transport of ABA and ABA-glucosyl ester(ABA-GE),respectively.We first revealed that ABCG16 contributes to osmotic stress tolerance via ABA signaling.More specifically,ABCG16 induces cellular ABA efflux in both yeast and plant cells.Using FRET analysis,we showed that ABCG16 forms oblig-atory homodimers for ABA export activity and that the plasma membrane-resident ABCG16 homodimers specifically respond to ABA,undergoing notable conformational changes.Furthermore,we demonstrated that ABCG16 heterodimerizes with ABCG25 at the endoplasmic reticulum(ER)membrane and facilitates the ER entry of ABA-GE in both Arabidopsis and tobacco cells.The specific responsiveness of the ABCG16-ABCG25 heterodimer to ABA-GE and the superior growth of their double mutant support an inhib-itory role of these twoABCGs in early seedling establishment via regulation of ABA-GE translocation across the ER membrane.Our endomembrane-specific analysis of the FRET signals derived from the homo-or heterodimerized ABcG complexes allowed us to link endomembrane-biased dimerization to the transloca-tion of distinct substrates by ABcG transporters,providing a prototypic framework for understanding the omnipotence of ABcG transporters in plant development and stress responses.

OsPDR20 is an ABCG metal transporter regulating cadmium accumulation in rice

Cadmium(Cd)is a non-essential toxic heavy metal,seriously posing high environmental risks to human health.Digging genetic resources relevant to functional genes is important for understanding the metal absorption and accumulation in crops and bioremediation of Cd-polluted environments.This study investigated a functionally uncharacterized ATP binding cassette transporter G family(ABCG)gene encoding a Pleiotropic Drug Resistance 20(PDR20)type metal transporter which is localized to the plasma membrane of rice.OsPDR20 was transcriptionally expressed in almost all tissues and organs in lifespan and was strongly induced in roots and shoots of young rice under Cd stress.Ectopic expression of OsPDR20 in a yeast mutant ycf1 sensitive to Cd conferred cellular tolerance with less Cd accumulation.Knockdown of OsPDR20 by RNA interference(RNAi)moderately attenuated root/shoot elongation and biomass,with reduced chlorophylls in rice grown under hydroponic medium with 2 and 10μmol/L Cd,but led to more Cd accumulation.A field trial of rice grown in a realistic Cd-contaminated soil(0.40 mg/kg)showed that RNAi plants growth and development were also compromised compared to wild-type(WT),with smaller panicles and lower spikelet fertility but little effect on yield of grains.However,OsPDR20 suppression resulted in unexpectedly higher levels of Cd accumulation in rice straw including lower leaves and culm and grain.These results suggest that OsPDR20 is actively involved in Cd accumulation and homeostasis in rice crops.The increased Cd accumulation in the RNAi plants has the potential application in phytoremediation of Cd-polluted wetland soils.

Maize MS2 encodes an ATP-binding cassette transporter that is essential for anther development

The anther cuticle and pollen exine play a critical role in male gametophyte development. The sporopollenin precursors and cuticular lipid monomers are transported to the surface of the microspores and the epidermis by lipid transport proteins(LTPs) and ATP-binding cassette G(ABCG) transporters for the formation of the pollen wall and anther cuticle, respectively. However, the function of ABCG transporters in maize anther development is unclear. Here, we cloned the MS2 gene from the maize male sterile2 mutant using map-based cloning and determined that it encodes an ABCG transporter. MS2 protein was experimentally confirmed to be located on the cell membrane. The quantitative real-time PCR(qRT-PCR)results showed that MS2 was ubiquitously expressed in all vegetative and reproductive tissues, whereas a high transcriptional level of MS2 was observed in anthers, especially at the young microspore stage. Gas chromatography-mass spectrometry(GC–MS) analysis showed decreased accumulation of cutin and wax components in ms2 anthers, indicating that MS2 plays a role in the transport of lipid molecules to anther cuticle and pollen exine. To our knowledge, MS2 is the first reported ABCG transporter gene that participates in anther development in maize.

Less and shrunken pollen 1(LSP1) encodes a member of the ABC transporter family required for pollen wall development in rice(Oryza sativa L.)

Pollen fertility is an agronomic trait that strongly influences rice yield. Recent studies have revealed that the development of the pollen wall is required for pollen fertility and is regulated by several genes. However, the mechanisms underlying pollen and pollen wall development in rice remain largely unknown. In the present study, a point mutation in a gene on chromosome 1 was identified that resulted in the production of less and shrunken pollen(LSP) and led to defects in pollen wall formation. This gene was named LSP1 and was found to encode a member of the adenosine triphosphate-binding cassette(ABC)transporter G subfamily, OsABCG3. Two other loss-of-function mutants of LSP1/OsABCG3,generated using CRISPR/Cas9 technology, showed the same male sterile phenotype. The LSP1/OsABCG3 gene showed a spatio-temporal expression pattern in the developing anthers, and is an ortholog of the Arabidopsis genes At ABCG1 and At ABCG16, which play an important role in pollen wall development. Mutation of LSP1/OsABCG3 affected the expression of several genes involved in pollen and pollen wall formation. These results suggest that LSP1/OsABCG3 is critical for normal pollen fertility and shed light on the molecular mechanisms underlying rice pollen wall development.

MET inhibitor tepotinib antagonizes multidrug resistance mediated by ABCG2 transporter:In vitro and in vivo study

Overexpression of ABCG2 transporter in cancer cells has been linked to the development of multidrug resistance(MDR), an obstacle to cancer therapy. Our recent study uncovered that the MET inhibitor,tepotinib, is a potent reversal agent for ABCB1-mediated MDR. In the present study, we reported for the first time that the MET inhibitor tepotinib can also reverse ABCG2-mediated MDR in vitro and in vivo by directly binding to the drug-binding site of ABCG2 and reversibly inhibiting ABCG2 drug efflux activity, therefore enhancing the cytotoxicity of substrate drugs in drug-resistant cancer cells. Furthermore, the ABCB1/ABCG2 double-transfected cell model and ABCG2 gene knockout cell model demonstrated that tepotinib specifically inhibits the two MDR transporters. In mice bearing drug-resistant tumors, tepotinib increased the intratumoral accumulation of ABCG2 substrate drug topotecan and enhanced its antitumor effect. Therefore, our study provides a new potential of repositioning tepotinib as an ABCG2 inhibitor and combining tepotinib with substrate drugs to antagonize ABCG2-mediated MDR.

Overexpression of the steroidogenic acute regulatory protein increases the expression of ATP-binding cassette transporters in microvascular endothelial cells(bEnd.3)

Objective:To determine the effect of steroidogenic acute regulatory protein(StAR) overexpression on the levels of adenosine triphosphate(ATP)-binding cassette transporter A1(ABCA1) and ATP-binding cassette transporter G1(ABCG1) in an endothelial cell line(bEnd.3).Methods:The StAR gene was induced in bEnd.3 cells with adenovirus infection.The infection efficiency was detected by fluorescence activated cell sorter(FACS) and fluorescence microscopy.The expressions of StAR gene and protein levels were detected by real-time polymerase chain reaction(PCR) and Western blot.The gene and protein levels of ABCA1 and ABCG1 were detected by real-time PCR and Western blot after StAR overexpression.Results:The result shows that StAR was successfully overexpressed in bEnd.3 cells by adenovirus infection.The mRNA and protein expressions of ABCA1 and ABCG1 were greatly increased by StAR overexpression in bEnd.3 cells.Conclusion:Overexpression of StAR increases ABCA1 and ABCG1 expressions in endothelial cells.