Calmodulin is one of the most well characterized proteins and a widely used
model system for calcium binding and large-scale protein conformational changes.
Its long central helix is usually cut in half when a target peptide is bound.
Here, two new crystal structures of calmodulin are presented, in which
conformations possibly representing the first steps of calmodulin conformational
collapse have been trapped. The central helix in the two structures is bent in
the middle, causing a significant movement of the N- and C-terminal lobes with
respect to one another. In both of the bent structures, a nearby polar side
chain is inserted into the helical groove, disrupting backbone hydrogen bonding.
The structures give an insight into the details of the factors that may be
involved in the distortion of the central helix upon ligand peptide binding.
