CXCR3 is a G-protein coupled receptor expressed principally on leukocytes, monocytes and epithelial cells; it is involved in leukocyte traffic, integrin activation, cytoskeletal changes and chemotactic migration, by binding to its classical ligands, CXCL-9/10/11 (1). Three splicing variants of CXCR3 are known: CXCR3a, the most common isoform, consisting of 368 amino acid residues; CXCR3b resulting from an alternative splicing of the CXCR3 mRNA with a 52 aa extended N-terminal domain when compared to the isoform a, while CXCR3-alt is a significantly truncated variant not involved in classical ligand binding (1). CXCL10, the interferon-?-inducible protein (IP-10) belongs to the CXC family of chemokines and acts as an immunoinflammatory mediator, inhibits angiogenesis and displays antitumor properties (2). Several studies indicated that the CXCR3 N-terminal domain plays a key role in determining binding affinity, receptor selectivity, and also in regulating allosteric signalling through the receptor (3). Moreover, tyrosine sulfation in chemokine receptors is emerging as a post-translational modification that contributes substantially to ligand binding (2). Tyr 27 and 29, two of the N- terminal tyrosine can be sulfonated. Finally, Kleist and co-workers hypothesize a "two-step" model, where receptor binding and activation can be dissociated (4). In the first step, the chemokine binds to the N-term domain of the receptor. In the second, residues on IP10 N-terminal bind to the binding cavity on the TM domain of the receptor and induce the allosteric communication to the cytosol. In this work, we analyse by a supervised molecular dynamics (SuMD) (5) simulation, the binding mechanism of IP10 on CXCR3. We explored the binding site in order to evaluate the residue- residue interactions between the chemockine and its receptor. Starting from the same equilibrated system, 3 runs of supervised MD were performed using the same conditions. A residue selection from the CXCR3 N-term domain and all the Extra-Cellular Loop (ECL) were considered for the binding interface, instead the entire chemokine was treated as ligand in order to perform the supervision on the distance between the receptor (CXCR3) and the ligand (CXCL10). They resulted with consistent similarity in both ligand-receptor contacts and interaction energies. Simulations didn't had identical durations (30.8 ns for simulation 1, 31.2 ns for simulation 2 and 36.6 ns for simulation 3), but all of them reached the ligand correctly positioned in the binding site in 15ns. Simulations 2 and 3 presented overlapping situations of decreasing energies related to minor R-L distances and of contacts, involving both residues from the CXCR3-N- term and the Extra-cellular ?-sheet, which has been described as an important player for the receptor activation. The next step will be the evaluation of the effect of tyrosine sulfation (Y27 and Y29) on binding mode and affinity.
CXCL10 binding mode to CXCR3 isoforms: different behaviors
Moscatelli M;Milanesi L;Mezzelani A;Chiappori F
2019
Abstract
CXCR3 is a G-protein coupled receptor expressed principally on leukocytes, monocytes and epithelial cells; it is involved in leukocyte traffic, integrin activation, cytoskeletal changes and chemotactic migration, by binding to its classical ligands, CXCL-9/10/11 (1). Three splicing variants of CXCR3 are known: CXCR3a, the most common isoform, consisting of 368 amino acid residues; CXCR3b resulting from an alternative splicing of the CXCR3 mRNA with a 52 aa extended N-terminal domain when compared to the isoform a, while CXCR3-alt is a significantly truncated variant not involved in classical ligand binding (1). CXCL10, the interferon-?-inducible protein (IP-10) belongs to the CXC family of chemokines and acts as an immunoinflammatory mediator, inhibits angiogenesis and displays antitumor properties (2). Several studies indicated that the CXCR3 N-terminal domain plays a key role in determining binding affinity, receptor selectivity, and also in regulating allosteric signalling through the receptor (3). Moreover, tyrosine sulfation in chemokine receptors is emerging as a post-translational modification that contributes substantially to ligand binding (2). Tyr 27 and 29, two of the N- terminal tyrosine can be sulfonated. Finally, Kleist and co-workers hypothesize a "two-step" model, where receptor binding and activation can be dissociated (4). In the first step, the chemokine binds to the N-term domain of the receptor. In the second, residues on IP10 N-terminal bind to the binding cavity on the TM domain of the receptor and induce the allosteric communication to the cytosol. In this work, we analyse by a supervised molecular dynamics (SuMD) (5) simulation, the binding mechanism of IP10 on CXCR3. We explored the binding site in order to evaluate the residue- residue interactions between the chemockine and its receptor. Starting from the same equilibrated system, 3 runs of supervised MD were performed using the same conditions. A residue selection from the CXCR3 N-term domain and all the Extra-Cellular Loop (ECL) were considered for the binding interface, instead the entire chemokine was treated as ligand in order to perform the supervision on the distance between the receptor (CXCR3) and the ligand (CXCL10). They resulted with consistent similarity in both ligand-receptor contacts and interaction energies. Simulations didn't had identical durations (30.8 ns for simulation 1, 31.2 ns for simulation 2 and 36.6 ns for simulation 3), but all of them reached the ligand correctly positioned in the binding site in 15ns. Simulations 2 and 3 presented overlapping situations of decreasing energies related to minor R-L distances and of contacts, involving both residues from the CXCR3-N- term and the Extra-cellular ?-sheet, which has been described as an important player for the receptor activation. The next step will be the evaluation of the effect of tyrosine sulfation (Y27 and Y29) on binding mode and affinity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
