Changes in MCL structures resulting from ligand binding. A) Trimers of the MCLC structure in the open (PDB 4L7Z) and closed conformation (PDB 4L80) without and with bound substrates; respectively. The view is from the top along the 3-fold rotation axis. A Tris molecule is positioned at this axis buried in the protein. Ligands are depicted as stick models. B) An overlay of monomers of the MCLC structures in the closed form (orange) and open form (grey). The C-terminal lid domain is rotated about 30° resulting in a shift of approximately 16 Å at its extremity. C) Comparison of quaternary structures. Trenches at the surface that are present in the closed conformation between the N-termini and the C-terminal lid domains are completely covered by the lid domain in the open conformation. Therefore, the N-termini seem to limit the vertical movement of the lid domain. The different orientation and reduced size of the N-termini in MCLR are responsible for an opening in the hexameric assembly that allows access to a central cavity. A similar cavity is also present in MCLC, but the access is obstructed.