Overall, the E2(AzTG-4) / E2(AzTG-6) structures are similar to SERCA in complex with underivatised TG (PDB ID: 2C8L), with an r.m.s.d. of 1.36 Å / 1.33 Å over all atoms and 0.94 Å / 0.84 Å over mainchain atoms. AzTG-4 and AzTG-6 are found bound to the TG-binding site located at the transmembrane (TM) domain between the M3, M5, and M7 helices (Figure ). In both structures, the azobenzene moiety resides within the TM domain itself -a rotation of the Leu253 sidechain (M3 helix) allows the photoswitch to insert between the M3, M4, and M5 helices (Figure ). This is consistent with findings from previous structures of SERCA complexed with other TG derivatives possessing O-8 extensions with bulky terminal groups, such as the E2(Boc-12ADT) structure (PDB ID: 2BY4), and appears to be the general binding mechanism for this class of compounds. Interestingly, however, the AzTG-4 and AzTG-6 photoswitches occupy distinct hydrophobic pockets. In the case of AzTG-4, the azobenzene moiety is situated within a shallow cavity outlined by Leu249, Leu253 and Phe256 from M3, Ile315 from M4, Ile761 and Ile765 from M5, and Leu828 from the M6-M7 loop (Figure ). In addition to the rotation associated with the Leu253 side chain, the bulk of the azobenzene group imparts an upward displacement on the side chain of Leu249, though this does not cause significant distortion of the M3 helix itself (Figure ). In contrast, the AzTG-6 azobenzene moiety lies further inside the TM domain, within a phospholipid-binding region outlined by Leu61, Val62 and Leu65 from M1, Leu253 and Phe256 from M3, as well as Ile307, Pro308, Leu311, and Pro312 from M4 (Figure ). AzTG-6 distinguishes itself from AzTG-4 through the insertion of the azobenzene group below Leu253, where its steric bulk does not affect the position of Leu249 (Figure ). In both cases, interactions between the photoswitch and surrounding residues are purely hydrophobic in nature and therefore are expected to be dominated by Van der Waals forces.