–Dr. Ayan Raichaudhuri & Anuska Sen, Team BioXone
The ATP-binding cassette (ABC) protein superfamily present in most eukaryotic and prokaryotic organisms, form one of the largest protein families known, the majority of whose members are membrane proteins (ABC “transporters”). These transporters use ATP to pump molecules across a membrane and so are known to be a type of ATPases. While ATPases are generally responsible for the transport of metal ions or protons, on the other hand, ABC transporters have a larger coverage of substrates they transport, ranging from small inorganic as well as organic molecules, such as amino acids, sugars, vitamins, and metal clusters to larger organic compounds such as peptides, lipid molecules, oligonucleotides, polysaccharides, etc.
Structural aspects of plant ABC transporters:
In general, ATP Binding Cassette (ABC) transporter has a typical construction consisting of at least four domains: – two nucleotide-binding domains/folds (NBD or NBF) and two transmembrane domains (TMD). Extra domains can be added to these core elements to grant regulatory functions. A periplasmic binding protein is required for the delivery of the ligand to prokaryotic importers of this family.
The NBDs contain conserved motifs involved in ATP binding and hydrolysis. Both NBDs contribute to the formation of two ATP-binding sites at their interface. The NBD (also known as the ATPase domain) contains a Walker motif A and Walker motif B on the two ends of an ABC signature motif. The one component of the NBD domain which distinguishes the ABC-proteins from other ATP-binding proteins is the ABC signature motif (also known as C motif or LSGGQ motif), located between the two Walker boxes Two other loops known as the H loop and Q loop are also present.
ABCC Transportes- a special class of ABC Transporters
A special type of ABC transporters known as Multidrug resistance protein (MRPs) or ABCC transporters is one of the highly represented subfamilies of ABC transporters. Plant MRPs are known to transport various glutathione conjugates across membranes. An ABCC transporter protein in Arabidopsis thaliana, namely AtMRP1 is already known to be involved in the vacuolar storage of folates. This transporter also showed increased tolerance towards arsenite As(III) stress in yeast. CKIIII/CKII mediated phosphorylation. An interesting observation is that structural similarity and similar names among different ABCC transporters do not ensure their functional similarity. For example, AtABCC7 does not necessarily share the same functions with OsABCC7 or HsABCC7.
In plants, the ABCC (or MRP) transporters are known to be involved in a wide variety of functions such as abiotic and biotic stress tolerance by metal detoxification or cleavage, vacuolar pumps, regulation of ion-channels, transport of catabolites, herbicide resistance, and other mechanisms. ABCC (or MRP) transcripts are known to show specificity in their response to oxidative stress. Results showed that AtMRP1 is important for the vacuolar accumulation of antifolates as well as tolerance against arsenic, both of which involved phosphorylation in the serine triads at the C terminal NBD of AtMRP1.
In Triticum aestivum, the genes TaABCC3, TaABCC4, TaABCC11, and TaABCC14 were significantly up-regulated when exposed to cadmium. Nevertheless, out of all these, TaABCC3 is suggested to be an important gene in conferring tolerance against heavy-metal accumulation. OsABCC1, the homolog of AtABCC1 in rice is instrumental in arsenic detoxification by sequestering it in the vacuoles and reducing its accumulation in rice grains. ABCC transports are also known to be instrumental in heavy metal stress mitigation in a wide variety of plants.
Future prospects
The genome-wide characterizations of ABC-type transporters in different plant species such as Arabidopsis, rice, wheat, grapes, soybean, and many other plant systems have identified a large number of genes to be present in the ABCC (or MRP) subfamily, which makes it the third-largest in the family. Thus, it can be suggested that the involvement of ABCC transporters in heavy metal stress tolerance will be of quite a significant benefit to the plant. Successful implementation of molecular techniques involving ABCC-transporters on a larger scale would surely be a boon for the food industry.
Also read: Gut microbiota and the potential risk of colon cancer
Suggested Articles:
- Raichaudhuri, Ayan. (2016). Arabidopsis thaliana MRP1 (AtABCC1) nucleotide binding domain contributes to arsenic stress tolerance with serine triad phosphorylation. Plant Physiology and Biochemistry. 108. http://10.1016/j.plaphy.2016.07.005.
2. Raichaudhuri, Ayan & Peng, Mingsheng & Naponelli, Valeria & Chen, Sixue & Sanchez-Fernandez, Rocio & Gu, Honglan & Gregory, Jesse & Hanson, Andrew & Rea, Philip. (2009). Plant Vacuolar ATP-binding Cassette Transporters That Translocate Folates and Antifolates in Vitro and Contribute to Antifolate Tolerance in Vivo. Journal of Biological Chemistry. 284. 8449-8460. http://10.1074/jbc.M808632200.
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