Calreticulin enhances the secretory trafficking of a misfolded α-1-antitrypsin

Abstract

Alpha-1-antitrypsin (AAT) regulates the activity of multiple proteases in the lungs and liver. A E342K mutant of AAT called ATZ forms polymers present at only low levels in the serum that induce intracellular protein inclusions, causing lung emphysema and liver cirrhosis. An understanding of factors that can reduce the intracellular accumulation of ATZ is of great interest. We now show that calreticulin (CRT), an endoplasmic reticulum (ER) glycoprotein chaperone, promotes the secretory trafficking of ATZ, enhancing the media to cell ratio. This effect is more pronounced for ATZ than with AAT, and is only partially dependent on the glycan-binding site of CRT, which is generally relevant to substrate recruitment and folding by CRT. The CRT-related chaperone calnexin (CNX) does not enhance ATZ secretory trafficking, despite the higher cellular abundance of CNX-ATZ complexes. CRT deficiency alters the distributions of ATZ-ER chaperone complexes, increasing ATZ-BiP binding and inclusion body formation, and reducing ATZ interactions with components required for ER-Golgi trafficking, coincident with reduced levels of the protein transport protein Sec31A in CRT-deficient cells. These findings indicate a novel role for CRT in promoting the secretory trafficking of a protein that forms polymers and large intracellular inclusions. Inefficient secretory trafficking of ATZ in the absence of CRT is coincident with enhanced accumulation of ER-derived ATZ inclusion bodies (IB). Further understanding of the factors that control the secretory trafficking of ATZ and their regulation by CRT could lead to new therapies for lung diseases linked to AAT deficiency.

Mechanism of mutant calreticulin-mediated activation of the thrombopoietin receptor in cancers

“…The sample was analyzed by SDS-PAGE under reducing and nonreducing conditions. Bands corresponding to monomers and dimers from the nonreducing gel were subjected to mass spectrometric analyses as described previously ( Mohan et al, 2020 ). Briefly, the excised gels were resuspended in 50 µl of 0.1 M ammonium bicarbonate buffer (pH 8).…”