Structure of Calreticulin C-terminal Domain Is Modulated by Physiological Variations of Calcium Concentration

Calreticulin is an abundant endoplasmic reticulum resident protein that fulfills at least two basic functions. Firstly, due to its ability to bind monoglucosylated high mannose oligosaccharides, calreticulin is a central component of the folding quality control system of glycoproteins. On the other hand, thanks to its capacity to bind high amounts of calcium, calreticulin is one of the main calcium buffers in the endoplasmic reticulum. This last activity resides on a highly negatively charged domain located at the C terminus. Interestingly, this domain has been proposed to regulate the intracellular localization of calreticulin. Structural information for this domain is currently scarce. Here we address this issue by employing a combination of biophysical techniques and molecular dynamics simulation. We found that calreticulin C-terminal domain at low calcium concentration displays a disordered structure, whereas calcium addition induces a more rigid and compact conformation. Remarkably, this change develops when calcium concentration varies within a range similar to that taking place in the endoplasmic reticulum upon physiological fluctuations. In addition, a much higher calcium concentration is necessary to attain similar responses in a peptide displaying a randomized sequence of calreticulin C-terminal domain, illustrating the sequence specificity of this effect. Molecular dynamics simulation reveals that this ordering effect is a consequence of the ability of calcium to bring into close proximity residues that lie apart in the primary structure. These results place calreticulin in a new setting in which the protein behaves not only as a calcium-binding protein but as a finely tuned calcium sensor.