D. drosophila AHR receptor PAS-B domain

Jackson Newell '24

Contents:

I. Introduction



 The drosophila aryl hydrocarbon receptor (dAHR) is a member of the basic helix-loop-helix Per-ARNT-Sim family of transcription factors. These transcription factors are observed in many organisms, and have important regulatory functions in physiological and developmental pathways (Crews et al. 1998). Aryl hydrocarbon receptors specifically are involved in toxin processing and bind toxic ligands. Once a toxic ligand is bound, AHR can enact a wide range of physiological responses to mediate the effects of toxic compounds.(Petrulis et al. 2003). AHR proteins are also of interest in cancer research as their overexpression has been observed in certain tumors (Xue et al. 2018).

dAHR is comprised of a helix-loop-helix domain responsible for DNA binding and two PAS domains (A and B) which participate in dimerization and trancription activation. The dAHR PAS-B domain functions as the ligand binding domain and is the site of heterodimerization with ARNT.

Once a ligand is bound, AHR migrates from the cytosol to the nucleus. Here, AHR forms a heterodimer with aryl hydrocarbon receptor nuclear translocator (ARNT). The AHR ARNT complex can then regulate transcription activity to enact responses to toxic compounds (Figure. 1).


Fig. 1. AHR transcriptional regulation pathway. By the Powell lab group.


II. General Structure

the dAHR PAS-B domain is arranged as beta-A-beta-B-alpha-C-alpha-D-alpha-E-beta-F-beta-G-beta-H-beta-I-alpha-J


Fig. 2. Schematic showing the structure of dAHR PAS-B. From Shuyan et al. 2022.


Three dimensionally the protein resembles a catchers mitt, with six beta-sheets

which form the and three alpha-helices which for the opposing Ligand binding occurs at the center of this sturcture, in the "palm". This is a typical alpha/beta PAS fold, which is shared among many PAS proteins

*Note: aD is not shown as a ribbon in the PDB file. This could be because it is 3 residues long and thus does not complete a whole turn, which is 3.6 residues in length.

III. Ligand Binding
Fig. 3. Structure of alpha-NF, the first and only ligand crystallized with AHR PAS-B.


AHR proteins can bind a variety of ligands. Often they bind toxic ligands, the first step of enacting an immune response to the presence of toxins. Once a ligand is bound, AHR migrates from the cytosal to the nucleus.

To bind alpha-NF, dAHR undergoes a confirmational change in the BG and BF region, the "palm" of the catchers mitt. Here, space is made for binding alpha-NF as M331 rotates about 90 degrees, fliping away from and opening the cavity.

alpha-NF binds to dAHR in the middle of the PAS-B cavity, primarily held by with residues F271, H275, F279, L281, M284, L292, G304, Y305, V316, H320, Y334, Y336, L346, H366.

via the G304 carbonyl oxygen and the Y334 side chain also helps to bind alpha-NF.

IV.  ARNT Heterodimer Complex

After entering the nucleus, ligand bound dAHR PAS-B interacts with the aryl hydrocarbon receptor nuclear translocator (ARNT) to form a heterodimer. ARNT is required for DNA binding, as it is essential for binding the XRE regulatory sequence (Reyes et al. 1992). This heterodimer can then begin transcriptional activation, in this way it is able to enact a wide variety of immune responses to toxins.

This dimerization is primarily mediated by Residues Y305, Y310, and L313 of dAHR PAS-B play a predominant role in facilitating this interaction by interacting with residues N448, D377, and Y456 of ARNT respectively.


also helps to mediate this process. Residues R336, N448, Y456, of ARNT form H-bonds with W343, D278, D306 of dAHR PAS-B respectively.

V. Real World Application

AHR is of great interest in cancer immunotherapy research due to its observed overexpression in certain types of tumors. It is unkown if it acts as an oncogene, an immunosuppressive effector, or both. In the suggested oncogene model it is thought to have an important role in various stages of tumorigenesis, contributing to increased proliferation, growth rate, and tissue invasion of tumor cells. In the suggested immunosupressive model it is thought to play a supressive role in the function and growth of various immune cells such as B cells and T cells (Xue et al. 2018).

Due to its observed effects in the tumor environment, it is thought that the selective suppression of AHR could result in a novel cancer immunotherapy treatment.

VI. References

Crews, S. T. (1998). Control of cell lineage-specific development and transcription by bHLH PAS proteins. Genes and development, 12(5), 607-620.

Dai, S., Qu, L., Li, J., Zhang, Y., Jiang, L., Wei, H., ... & Chen, Y. (2022). Structural insight into the ligand binding mechanism of aryl hydrocarbon receptor. Nature communications, 13(1), 1-12.

Petrulis, J. R., Kusnadi, A., Ramadoss, P., Hollingshead, B., and Perdew, G. H. (2003). The hsp90 co-chaperone XAP2 alters importin beta recognition of the bipartite nuclear localization signal of the Ah receptor and represses transcriptional activity. Journal of Biological Chemistry, 278(4), 2677-2685.

Reyes, H., Reisz-Porszasz, S., & Hankinson, O. (1992). Identification of the Ah receptor nuclear translocator protein (Arnt) as a component of the DNA binding form of the Ah receptor. Science, 256(5060), 1193-1195.

Xue, P., Fu, J., & Zhou, Y. (2018). The aryl hydrocarbon receptor and tumor immunity. Frontiers in immunology, 9, 286.

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