Myofibrillar myopathy-associated KY protein promotes muscle fibre size rescue independent of its predicted catalytic triad

Mutations in the Ky gene are the underlying cause of Myofibrillar Myopathy-7 (MFM-7), a rare progressive muscle weakness disease of childhood onset. A defining characteristic of the KY protein is the presence of a conserved transglutaminase-like domain, but unequivocal evidence of its enzymatic function remains to be established. To investigate the functional relevance of the predicted KY catalytic triad we use here in vitro enzymatic assays, structural modeling and in vivo rescue experiments in ky/ky mice. While structural modelling shows a striking conservation of the catalytic pocket architecture, our results show that recombinant KY proteins showed no detectable enzymatic activity under the assay conditions used. Moreover, while deletion of transglutaminase-like domain prevents phenotype rescue, replacements of the predicted catalytic residues do not impair the protein's ability to rescue fibre size in ky/ky muscle, indicating that the predicted catalytic residues are dispensable for fibre size rescue in these assays. Proteomic analyses identified KY-associated protein complexes involved in protein quality control, including core components of the Chaperone-Assisted Selective Autophagy machinery. In agreement, basal autophagic flux is significantly reduced in both KY-deficient C2C12 cells and ky/ky muscle fibres. Collectively, our data suggests that the TGN/PROT domain facilitates critical molecular associations at the sarcomeric Z-disc through a mechanism independent of catalysis and that impaired autophagic flux may contribute to the muscle phenotype.