Calreticulin mutations affect its chaperone function and perturb the glycoproteome

Calreticulin (CALR) is an endoplasmic reticulum (ER)-retained chaperone that assists glycoproteins in obtain-ing their structure. CALR mutations occur in patients with myeloproliferative neoplasms (MPNs), and the ER retention of CALR mutants (CALR MUT) is reduced due to a lacking KDEL sequence. Here, we investigate the impact of CALR mutations on protein structure and protein levels in MPNs by subjecting primary patient sam-ples and CALR-mutated cell lines to limited proteolysis-coupled mass spectrometry (LiP-MS). Especially gly-coproteins are differentially expressed and undergo profound structural alterations in granulocytes and cell lines with homozygous, but not with heterozygous, CALR mutations. Furthermore, homozygous CALR mu-tations and loss of CALR equally perturb glycoprotein integrity, suggesting that loss-of-function attributes of mutated CALR chaperones (CALR MUT) lead to glycoprotein maturation defects. Finally, by investigating the misfolding of the CALR glycoprotein client myeloperoxidase (MPO), we provide molecular proof of protein misfolding in the presence of homozygous CALR mutations.

Automated summary

This study investigates the impact of CALR mutations on protein structure and protein levels in MPNs. It is found that homozygous CALR mutations lead to profound structural alterations and perturbation of glycoprotein integrity.