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. Author manuscript; available in PMC: 2026 Feb 1.
Published in final edited form as: Biochemistry. 2026 Jan 2;65(2):137–148. doi: 10.1021/acs.biochem.5c00719

Figure 3.

Figure 3.

CBD structure and putative ligand binding. (A) Top and side views of a locally refined cryo-EM density map (EMD-44557) of the 2-fold pore of the Sg Enc shell from S. griseus in the presence of 20 mM cAMP.9 HK97-fold shell density is shown in blue, CBD density is shown in green, and MBD density is shown in purple. (B) Structure of the Sg Enc CBD (PDB: 9BHU). A Ca2+ ion is shown as a green sphere within the MBD. (C) Structure of the CBD from E. coli CAP in complex with cAMP (PDB: 1G6N).72 (D) A close-up of the Sg Enc CBD binding pocket. Sequence conservation between residues of other Family 2B CBDs is highlighted in color according to ConSurf conservation score.73 Residues labeled with asterisks are conserved cAMP-interacting residues between the Sg Enc CBD and the E. coli CAP CBD. W176 is shown in bold. Adapted with open access permission via a creative common license (https://creativecommons.org/licenses/by/4.0/).9 (E) A close-up of the CAP binding pocket in complex with cAMP (yellow). The cAMP-coordinating R82 residue is shown in bold. Adapted with open access permission via a creative common license (https://creativecommons.org/licenses/by/4.0/).9 (F) Ligand binding to external CBDs may control the state of the 2-fold pore (open or closed). Upon ligand binding to the open-state shell (left), the 2-fold pores and CBDs may change conformation and assume a closed state (right) in which the encapsulated TC cargo is made inactive by restricting substrate access to the shell interior. (G) Different putative CBD-binding ligands.