Membrane-cytoskeleton linker. May participate in the maintenance/targeting of ion channels and cell adhesion molecules at the nodes of Ranvier and axonal initial segments. In skeletal muscle, required for costamere localization of DMD and betaDAG1 (By similarity). Regulates KCNA1 channel activity in function of dietary Mg(2+) levels, and thereby contributes to the regulation of renal Mg(2+) reabsorption (By similarity). May be a constituent of a NFASC/NRCAM/ankyrin G complex. Interacts with RHBG. Directly interacts with DMD and betaDAG1; this interaction does not interfere with DMD-binding and is required for DMD and betaDAG1 retention at costameres. Interacts (via N-terminal ANK repeats) with SCHIP1 isoform 7 (via C-terminus); this interaction is required for the localization at axon initial segments (AISs) and nodes of Ranvier (NRs) (By similarity). Interacts with PLEC and FLNC (PubMed:21223964). Isoform 7 interacts with PLEC and FLNC through its muscle-specific C-terminal sequence. Interacts with KCNA1; this inhibits channel activity (By similarity). Interacts (via ANK repeats) with IQCJ-SCHIP1; required for IQCJ-SCHIP1 localization at axon initial segments (AIS) and nodes of Ranvier (By similarity). Interacts with SCHIP1 (By similarity). Interacts with SCN5A (By similarity). When transfected in root dorsal ganglia, predominantly located in the axolemma of the axon proximal segments (PubMed:9744885). Also associated with the plasma membrane in neuron cell bodies, although at a lower level than in the axon proximal segment (PubMed:9744885). Isoform 1 is restricted to the axolemma of the axon proximal segment. Isoforms containing the muscle-specific 76 amino-acid insertion localize to the sarcolemma and on the postsynaptic membrane of neuromuscular junctions (PubMed:11796721). Expressed at highest levels in brain and testis, followed by skin, kidney, liver and spleen. Isoforms 2, 3, 4, 5, 6 and 7 may be specifically expressed in muscle tissues, including heart and skeletal muscle (extensor digitorum longus) (at protein level) (PubMed:11796721). The 76 amino-acid long sequence from Asp-1888 to Val-1963 is encoded by a muscle-specific exon. The 76 amino-acid long sequence from Asp-1710 to Val-1785 is encoded by a muscle-specific exon. The 76 amino-acid long sequence from Asp-1077 to Val-1152 is encoded by a muscle-specific exon. The 76 amino-acid long sequence from Asp-1060 to Val-1135 is encoded by a muscle-specific exon. The 76 amino-acid long sequence from Asp-881 to Val-956 is encoded by a muscle-specific exon. The 76 amino-acid long sequence from Asp-864 to Val-939 is encoded by a muscle-specific exon.