Calreticulin

Calreticulin is a protein that binds Ca2+ ions (a second messenger molecule in signal transduction), rendering it inactive. The Ca2+ is bound with low affinity, but high capacity, and can be released on a signal (see inositol triphosphate). Calreticulin is located in storage compartments associated with the endoplasmic reticulum.

Calreticulin is also known as calregulin, CRP55, CaBP3 and calsequestrin-like protein. The term "Mobilferrin" is considered to be the same as calreticulin by some sources.

Calreticulin binds to misfolded proteins and prevents them from being exported from the Endoplasmic reticulum to the Golgi apparatus.

Mechanism: A similar quality control chaperone, calnexin, performs the same service for soluble proteins as does calreticulin. Both proteins, Calnexin and calreticulin, have the function of binding to oligosaccharides containing terminal glucose residues thereby targeting them for degradation. In normal cellular function, trimming of glucose residues off of the core oligosaccharide added during N-linked glycosylation is a part of protein processing. If "overseer" enzymes note that residues are misfolded, proteins within the RER will re-add glucose resides so that other Calreticulin/Calnexin can bind to these proteins and prevent them from proceeding to the Golgi. This leads these aberrently folded proteins down a path whereby they are targeted for degradation rather than utility.