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. 2025 Apr 30;198(1):kiaf146.
doi: 10.1093/plphys/kiaf146.

Message hidden in α-helices-toward a better understanding of plant ABCG transporters' multispecificity

Wanda Biała-Leonhard  1 , Aleksandra Bigos  2   3 , Jan Brezovsky  2   3 , Michał Jasiński  1
Affiliations

Message hidden in α-helices-toward a better understanding of plant ABCG transporters' multispecificity

Wanda Biała-Leonhard et al. Plant Physiol. .
No abstract available

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Conflict of interest statement

Conflict of interest statement. None declared.

Figures

Figure 1.
Figure 1.
Comparison of ABCG's transported and nontransported molecules. Molecules tested in transport assays for chosen examples of plant ABCG proteins are presented as structural formulas. Differentiating elements within each group of molecules are highlighted in colors: AtABCG36—pink, AtABCG25—orange, AtABCG29—yellow, and MtABCG46—green and blue.
Figure 2.
Figure 2.
A schematic representation of the identified molecular determinants in plant ABCG transporters. NBDs and TMDs are tinted yellow and blue, respectively. Within TMDs, particular elements are highlighted: (1) helices forming the cavity from both halves are in dark blue and light blue, respectively. For the half-size transporters, helices are numbered in dark grey and numbered sequentially, with (‘) for the second half. For the full-size ABCG transporters, helices are numbered in light grey and the numbering preserves the order within a single polypeptide; (2) residues corresponding to substrate selectivity are indicated by colored dots and are listed on the right panel; and (3) regions corresponding to the extracellular (dark red circle) and cytoplasmic (dark blue circle) gates, respectively.
Figure 3.
Figure 3.
Comparative analysis of ABCG protein cavities. A) Cross-sections of ABCG transporters (AtABCG25, AtABCG16, AtABCG16-AtABCG25 heterodimer, AtABCG36, AtABCG37, MtABCG46, HsABCG1, HsABCG2, ScPDR5) show the architecture of cavities in their transmembrane regions (light green). The protein names are followed by PDB IDs for crystal structures or AF3 for AlphaFold3-predicted structures (Abramson et al. 2024). All structures are shown in their apo-state in the inward-facing open conformation except AtABCG16 (inward-facing closed). The crystal structure of HsABCG2 originally included imatinib, but the ligand was removed before cavity analyses. B) The accompanying table summarizes bottleneck diameters (calculated using CAVER 3.0; Chovancova et al. 2012), cavity volumes, and hydrophobicity scores (both calculated using mdpocket; Schmidtke et al. 2011). The bottleneck refers to the narrowest part of the access path leading to the central cavity. Hydrophobicity scores are calculated as the mean hydrophobicity of residues forming the cavity based on Monera et al. (1995) residue hydrophobicity scale, reaching values from −55 to 100 for polar aspartate and hydrophobic phenylalanine, respectively. ABCG transporters are ordered by volumes of their cavities.
Figure 4.
Figure 4.
Simplified translocation cycle (export). In its nonbound (apo) state, the protein is in an inward-facing conformation, ready for substrate binding. From the intracellular side, the molecule enters the cavity formed by TMDs. Subsequently, the binding of ATP to the NBD causes the NBD to close together and opens the translocation pathway on the extracellular side, altering the protein's conformation to outward-facing and releasing the substrate. After ATP hydrolysis, the transporter returns to an inward-facing conformation with a translocation pathway closed on both sides. ADP release then triggers the transporter's reset to the apo state. This model is based on recent studies by An et al. (2024), which provide evidence for all 4 stages, including the occluded one, supported by specific structures obtained for AtABCG16.
Figure 5.
Figure 5.
Conformational flexibility and spatial differences between MtABCG46 substrates. Structural differences between the transported substrate liquiritigenin and the nontransported substrate 7,4′-dihydroxyflavone are highlighted. Liquiritigenin adopts 4 distinct conformations while 7,4′-dihydroxyflavone has 3; their relative occurrence probabilities were calculated based on their relative energy (ΔE, kcal/mol) using CREST software (Pracht et al. 2024). Despite differing by only 1 bond, the conformational flexibility of these 2 molecules varies significantly. The conformers of liquiritigenin demonstrate greater diversity, with variations in the orientation of substituents. In contrast, 7,4′-dihydroxyflavone is more rigid and planar, with differences limited to hydroxyl group positioning and minor ring rotations. Coloring: carbon blue, oxygen red, hydrogen grey, structural formulas of both molecules are available in Fig. 1.
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References

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