Rescue of sarcoglycan mutations by inhibition of endoplasmic reticulum quality control is associated with minimal structural modifications.

Sarcoglycanopathies (SGP) are a group of autosomal recessive muscle disorders caused by primary mutations in one of the four sarcoglycan genes. The sarcoglycans (α-, β-, γ-, and δ-sarcoglycan) form a tetrameric complex at the muscle membrane that is part of the dystrophin-glycoprotein complex and plays an essential role for membrane integrity during muscle contractions. We previously showed that the most frequent missense mutation in α-sarcoglycan (p.R77C) leads to the absence of the protein at the cell membrane due to its blockade by the endoplasmic reticulum (ER) quality control. Moreover, we demonstrated that inhibition of the ER α-mannosidase I activity using kifunensine could rescue the mutant protein localization at the cell membrane. Here, we investigate 25 additional disease-causing missense mutations in the sarcoglycan genes with respect to intracellular fate and localization rescue of the mutated proteins by kifunensine. Our studies demonstrate that, similarly to p.R77C, 22 of 25 of the selected mutations lead to defective intracellular trafficking of the SGs proteins. Six of these were saved from ER retention upon kifunensine treatment. The trafficking of SGs mutants rescued by kifunensine was associated with mutations that have moderate structural impact on the protein.