|Figure 4: Molecular modelling of the CAR I281T mutation. The residue 281 (orange) is located
in the N-terminal part of helix H9 pointing to the loop between helices H8 and H9.
I281 is separated by >17Å from the SRC1 coactivator peptide (CoA; green) and >22Å
from the agonist CITCO, thus having no direct involvement in coactivator or ligand
binding. The N-terminal section of the loop forms a part of the heterodimerization
interface (red) with RXRα. Although I281 does not interact directly with RXRα, it
stabilizes the loop through hydrophobic interactions (dashed lines) with P270 and
V275 (grey). The I281T mutation introduces a shorter hydrophilic residue, which may
disrupt the interaction with V275 in the first rotamer (rotamer probabilities shown
in percentages), or disabling both hydrophobic interactions in the second rotamer.
The latter creates a possible clash with the backbone of R278 that destabilizes the
N-terminal part of H9. Both possibilities are likely to produce an allosteric effect
that may decrease heterodimerization of CAR with RXRα.