Fig. 2: Fab15H6.v4 binds to the distant LoopA of the HTRA1 trimer and stabilizes a non-competent conformation.

a Cryo-EM map of HTRA1^PD/SA trimer with three Fabs (Fab15H6.v4) bound to the exposed LoopA at a resolution of 3.3 Å. The monomers of the HTRA1^PD/SA trimer are in blue, yellow and red and Fab light and heavy chains are in pink and cyan, respectively. b Cryo-EM map of wildtype HTRA1^PD:Fab15H6.v4 complex at 3.3 Å resolution, in which the flexible constant regions (C_H1/C_L) of the Fab15H6.v4 were masked out during refinement. c Detailed view on the LoopA epitope and Fab15H6.v4 interaction. Critical residues are represented as sticks. d Ribbon representation of wildtype HTRA1^PD bound by Fab15H6.v4 in side view (top) and top view (bottom). The LoopA epitope is more than 30 Å away from the catalytic center. e Comparison of the Fab-bound wildtype HTRA1^PD (blue) with the competent apo-HTRA1 in the active conformation (wheat, pdb 3TJN_chainB) showing severe loop distortions in the catalytic center (loops in red). f Detailed view of the distorted active center loops (red) and catalytic residues of Fab-bound wildtype HTRA1^PD (blue) in comparison to the competent apo-HTRA1 conformation (wheat, pdb 3TJN_chainB); the catalytic serine S328 and H220 are out of position, the oxyanion hole (indicated by residue G326) is not formed and the L345 occludes the S1 pocket. g Comparison of Fab-bound HTRA1^PD (blue) and apo-HTRA1^PD (green, pdb 3TJN_chainA) with LoopD, Loop1, and Loop2 in similar, non-competent conformations (red, salmon).