Aciculin interacts with filamin C and Xin and is essential for myofibril assembly, remodeling and maintenance

Filamin C (FLNc) and Xin actin-binding repeat-containing proteins (XIRPs) are multi-adapter proteins mainly expressed in cardiac and skeletal muscles that play important roles in the assembly and repair of myofibrils and their attachment to the membrane. We identified the dystrophin-binding protein aciculin (PGM5), as a novel interaction partner of FLNc and Xin. All three proteins colocalize at intercalated discs of cardiac muscle and myotendinous junctions of skeletal muscle, while FLNc and aciculin also colocalize in mature Z-discs. Bimolecular fluorescence complementation experiments in developing cultured mammalian skeletal muscle cells demonstrate that Xin and aciculin also interact in FLNc-containing immature myofibrils and areas of myofibrillar remodeling and repair induced by electrical pulse stimulation (EPS). FRAP experiments show that aciculin is a highly dynamic and mobile protein. Aciculin knockdown in myotubes leads to failure in myofibril assembly, alignment and membrane attachment, and massive reduction in myofibril number. A highly similar phenotype was found upon depletion of aciculin in zebrafish embryos. Our results point to a thus far unappreciated but essential function of aciculin in myofibril formation, maintenance and remodeling.