ATP-binding cassette (ABC) transporters, a large class of transmembrane proteins, are widely found in organisms and play an important role in the transport of xenobiotics. Insect ABC transporters are involved in insecticide detoxification and Bacillus thuringiensis (Bt) toxin perforation. The complete ABC transporter is composed of two hydrophobic transmembrane domains (TMDs) and two nucleotide binding domains (NBDs).
1. Introduction
ATP-binding cassette (ABC) proteins comprise an extensive and variable transporter superfamily within P-loop motif and are found in all living organisms
[1,2,3][1][2][3]. Studies on ABC transporters began in the early 1970s with the biochemical characterization of substrate-binding protein-dependent transport in
Escherichia coli that was directly energized by hydrolysis of ATP
[4,5][4][5]. In 1982, cytoplasmic membrane-associated transporter genes in the histidine transport system of
Salmonella typhimurium (coded by the
hisP gene) and maltose-maltodextrin transport system of
E. coli (coded by the
malK gene) were cloned
[6,7][6][7]. Concurrently, in mammalian cells, the gene encoding permeability, glycoprotein (P-gp, a large glycosylated membrane protein related to multi-drug resistance) was identified and cloned in 1985
[8,9][8][9]. Eventually, substrate-binding transport proteins with ATP-binding subunits were found to constitute a large superfamily of transport proteins and termed ABC transporters in 1990
[10]. On the basis of differences in the ATP-binding sites among insect ABC transporters, the superfamily can be divided into eight subfamilies (ABCA to ABCH)
[11].
2. Structure and Mechanism of ABC Transporters
Structural models of ABC transporters are based on the crystal structure of different bacterial proteins that act as importers such as vitamin B12 transporter BtuCDF from
E. coli and exporters such as the multidrug exporter Sav1866 from
Staphylococcus aureus or related flippases such as MsbA lipid flippase from
E. coli [29,30,31][12][13][14]. On the basis of their architecture and biochemical activity, the ABC importers have been divided into type I and type II
[12,32,33][15][16][17]. The energy coupling factor (ECF) transporters, which differ structurally and functionally from other ABC importers, are sometimes considered as type III ABC importers
[34,35,36,37][18][19][20][21]. However, ABC importers have only been confirmed in prokaryotes, not in eukaryotes
[11,15][11][22]; therefore, in this review, we focus only on ABC exporters.
The structure of ABC transporters is highly conserved among most eukaryotic organisms, including insects. A functional ABC transporter is characterized by the presence of a P-type traffic ATPase, which comprises two cytosolic nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs)
[1,12,38][1][15][23] (
Figure 1A). The four domains of a functional transporter (2TMDs-2NBDs) are combined in a single polypeptide, forming a full transporter (FT), whereas a half transporter (HT) contains one TMD and one NBD, which are sometimes encoded as separate polypeptides and then fused into multidomain proteins. For ATP binding and hydrolysis, the HT must become a functional transporter by forming homo- or heterodimeric complexes. The NBD contains several highly conserved nucleotide-binding sequences such as the Walker A and B motifs, common in nucleotide-binding proteins (the Walker B motif also provides the catalytic base);
d-loop, which contains an aspartate residue and is responsible for forming a salt bridge; Q-loop, which contains a glutamate residue and acts as the attacking nucleophile in ATP hydrolysis; H motif, which has an invariant histidine active site that may be involved in maintaining the stability of the pre-hydrolytic state; and an α-helical region that has the ABC signature sequence (LSGGQ motif)
[12,13,39][15][24][25]. The ABC exporter fold, a prominent quaternary structure in TMDs in all ABC exporters, is characterized by 12 transmembrane helices and acts as a switch between different conformational changes and initiates substrate translocation
[12][15].
Figure 1. General structure of an ATP-binding cassette (ABC) full transporter (ABC exporter) and the ATP-switch model for the transport mechanism of ABC transporters. (
A) Typical ABC full transporter with two transmembrane domains (TMDs), TMD1 (green) and TMD2 (sky blue), and two nucleotide-binding domains (NBDs), NBD1 (red) and NBD2 (yellow). Each transmembrane domain (TMD) contains six transmembrane helices. The “long” multidrug-resistance associated proteins (MRPs) of the ABCC subfamily contains an additional TMD (TMD0) at the N terminus
[46][26]. (
B) The ATP-switch model
[14][27] includes (I) binding of the substrates (12-point blue circle) to the TMDs; (II) subsequent structural changes to the NBDs (red and yellow), hydrolysis of ATP (brown circles), followed by closed dimer formation of the NBDs and major conformational change in the TMDs, which initiates substrate translocation; (III) the ATP is hydrolyzed (gray circles), releasing ADP and Pi, and (IV) finally destabilization of the closed dimer restores its initial open dimer configuration for another new cycle. This figure is drawn by following the previous report of ABC transporter by Dermauw & Van Leeuwen
[14][27].
ABC transporters have a common mechanism for exporting substrates across the membrane by hydrolyzing ATP as a pump, but other models have been proposed for the ABC transporter mechanism based on structural and biochemical evidence, including the ATP-switch
[39][25], alternating site
[40][28], constant contact
[41[29][30],
42], and thermodynamic models
[43][31]. Among these models, the ATP-switch model provides a reasonable framework for the transport mechanism
[39,44,45][25][32][33] in which repeated communication between NBDs and TMDs occurs in both directions and involves only non-covalent conformational changes. The transport process is initiated by the binding of the substrate to the TMDs, and subsequent structural changes are transmitted to the NBDs, which include ATP-binding and closed dimer formation of the NBDs. Then the closed NBD dimer induces a substantial conformational change in the TMDs. This conformational change initiates translocation of the substrate through a rotation of the TMDs and opening toward the extracellular milieu. Finally, the ATP is hydrolyzed, releasing ADP and Pi and destabilizing the closed dimer conformation to restore its open dimer configuration for another new cycle
[39,44,45][25][32][33] (
Figure 1B).
3. ABC Transporter Subfamilies in Insects
In recent years, with the large-scale development of genome sequencing technology, the sequencing results have shown that ABC transporter genes are highly conserved in many insects (
Figure 2). Aside from some important discoveries on the function of some ABC transporters in insects, however, knowledge on the role and function of these proteins is still limited. ABC transporters of numerous important agricultural pests and model insects, such as
D. melanogaster,
B. mori,
Helicoverpa armigera and
Plutella xylostella, have been reported (
Table 1).
Figure 2. Phylogenetic tree based on amino acid sequences of 262 ABC transporters
(Supplementary Material 1) from several insects and humans. The sequences were aligned using MUSCLE. The evolutionary history was inferred using the neighbor-joining method and MEGA-X with 1000 bootstrap replicates. All positions with less than 95% site coverage were eliminated. Species codes: Ha,
Helicoverpa armigera; Bm,
Bombyx mori; Px,
Plutella xylostella; Hs,
Homo sapiens; Dm,
Drosophila melanogaster.
Table 1.
Distribution of genes among ABC transporter subfamilies for different arthropods and
Homo sapiens
.
Organisms |
A |
B |
C |
D |
E |
F |
G |
H |
Total |
References |
Homo sapiens |
12 |
11 |
12 |
4 |
1 |
3 |
5 |
0 |
48 |
[1] |
Drosophila melanogaster |
10 |
8 |
14 |
2 |
1 |
3 |
15 |
3 |
56 |
[1] |
Anopheles gambiae |
9 |
5 |
13 |
2 |
1 |
3 |
16 |
3 |
52 |
[47] | [34] |
Daphnia pulex |
4 |
7 |
7 |
3 |
1 |
4 |
24 |
15 |
65 |
[48] | [35] |
Pediculus humanus humanus |
2 |
6 |
5 |
2 |
1 |
3 |
13 |
6 |
40 | a |
[49] | [36] |
Apis melifera |
3 |
5 |
9 |
2 |
1 |
3 |
15 |
3 |
41 |
[50] | [37] |
Bombyx mori |
7 |
9 |
11 |
2 |
1 |
3 |
16 |
2 |
51 | b |
[50,51,52] | [37][38][39] |
Tribolium castaneum |
10 |
6 |
35 |
2 |
1 |
3 |
13 |
3 |
73 |
[20] | [40] |
Tetranychus urticae |
9 |
4 |
39 |
2 |
1 |
3 |
23 |
22 |
103 |
[53] | [41] |
Chrysomela populi |
5 |
Helicoverpa zea |
7 |
11 |
11 |
2 |
1 |
3 |
17 |
2 |
54 |
[ | 52 | ] | [ | 39 | ] |
Acyrthosiphon pisum |
11 |
9 |
16 |
2 |
1 |
4 |
19 |
9 |
71 |
[63,64] | [51][52] |