Type-VI intermediate filament (IF) which plays an important cytoskeletal role within the muscle cell cytoskeleton. It forms heteromeric IFs with desmin and/or vimentin, and via its interaction with cytoskeletal proteins alpha-dystrobrevin, dystrophin, talin-1, utrophin and vinculin, is able to link these heteromeric IFs to adherens-type junctions, such as to the costameres, neuromuscular junctions, and myotendinous junctions within striated muscle cells (By similarity). Interacts with DES, DMD, DTNA, TLN1, UTRN and VCL (By similarity). Isoform 1 and isoform 2 interact with GFAP and VIM. There are at least two distinct SYNM subpopulations, one in which SYMN interacts with DES within the Z-lines, and another in which it interacts with both DTNA and DES at the costamere. Isoform 2 and isoform 3 are detected in adult skeletal muscle, heart and bladder, whereas isoform 1 is only detected in adult bladder (at protein level). At 11.5 dpc, isoform 1 and isoform 2 are widely expressed in the developing nervous and vascular systems and are also found specifically associated with vimentin in endothelial cells. By 15 dpc, isoform 1, isoform 2 and isoform 3, are found coexpressed with neurofilament, peripherin and internexin in the peripheral nervous system (at protein level). In the developing embryo, isoform 2 is detected as early as 5 dpc, whereas isoform 1 is first observed at 9 dpc in the nervous system and mesodermic derivatives. Isoform 3 is observed later in neurons at 15 dpc. Up-regulated in reactive astrocytes following neurotrauma (at protein level). Belongs to the intermediate filament family.