Molecular and functional analysis of calcium binding by a cancer-linked calreticulin mutant

Calreticulin, a chaperone protein found within the endoplasmic reticulum (ER), is known to bind calcium ions. Mutations in the gene that encodes calreticulin, CALR, can lead to certain types of myeloproliferative neoplasms (MPNs). One such mutation, called CRT Del52, has been associated with altered ER calcium storage and signaling, which are thought to contribute to the development of MPNs.

Recombinant CRT proteins were used in experiments to investigate the calcium-binding properties of both the wild-type CRT and the CRT Del52 mutant. Isothermal titration calorimetry (ITC) revealed that both versions of the protein displayed low-affinity calcium-binding capabilities. Moreover, the residues at positions 340-349, which are shared by both the wild-type CRT and the CRT Del52 mutant, were found to play a role in calcium binding.

Further investigation into the effects of the CRT Del52 mutation on calcium levels within the ER and cytosol revealed that these levels were comparable between CRT knockout (CRT-KO) cells reconstituted with either the wild-type CRT or the CRT Del52 protein. The CRT-KO condition also led to an upregulation of various ER proteins that possess low-affinity calcium-binding sites or are involved in regulating ER calcium levels.

Overall, these findings suggest that the CRT Del52 mutation does not completely abolish the protein's ability to bind calcium ions, but rather allows for some level of calcium-binding capacity to be retained. Additionally, the study indicates that the absence of calreticulin protein may trigger a series of compensatory cellular changes that help maintain ER calcium homeostasis in the face of CRT deficiency.