a, Structure of the core in four perpendicular views shows the assembly and arrangement of the core layers. The bottom view (the same as the view of Fig. 4c) shows the surface electrostatic potential distributions of the core. The surfaces of αLCM molecules are circled. The surface electrostatic potentials were calculated in PyMol. Red, negative electrostatic potential; blue, positive electrostatic potential; grey, non-polar. b, A cartoon model of the core of a hemidiscoidal PBS (adapted from ref. 1). Note that the basal cylinder has four trimers. c, The inter-cylinder interaction in the core is symmetrical: trimers B1, A2 and A′1 interact with one another and trimers B2, A′2 and A1 interact with one another. The association is mediated by the interaction between three α-subunits (red circle) and two pairs of α−β subunits (black circle). d, Comparison of the three regions of L CM with the crystal structure of the Pfam00427 domain. The structures of the three regions of L CM (L CM -Reg1–3) are displayed as indicated and they plus the crystal structure of the Pfam00427 domain (PDB 3NPH) are aligned with one another. e–g, Close-up views of three PCB pockets of the β-subunits boxed by dashed lines in Fig. 4d: β1 (a), β2 (b) and ApcF (c). The bilins and their surrounding residues are shown in ball-stick representation and stick representation, respectively. Their cryo-EM densities (mesh) superimposed with the respective atomic models (stick) are shown in the lower panels. h, Structural superimposition of ApcF and βAPC shows that ApcF has an extension at the tip of the G–H helix hairpin. i, j, Cryo-EM densities (mesh) of the bilins in αLCM and the α3 subunit of core layer A2, and their surrounding residues, corresponding to Fig. 4f, superimposed with their respective atomic models (stick; coloured as in Fig. 4).