A nonerythroid isoform of protein 4.1R interacts with components of the contractile apparatus in skeletal myofibers

The similar to 80-kDa erythroid 4.1R protein is a major component of the er ythrocyte cytoskeleton, where it links transmembrane proteins to the underl ying spectrin/actin complexes. A diverse collection of 4.1R isoforms has be en described in nonerythroid cells, ranging from similar to 30 to similar t o 210 kDa. In the current study, we identified the number and primary struc ture of 4.1R isoforms expressed in adult skeletal muscle and characterized the localization patterns of 4.1R message and protein. Skeletal muscle 4.1R arrears to originate solely from the upstream translation initiation codon (AUG-1) residing in exon 2'. Combinations of alternatively spliced downstr eam exons generate an array of distinct 4.1R spliceoforms. Two major isofor m classes of similar to 105/110 and similar to 135 kDa are present in muscl e homogenates. 4.1R transcripts are distributed in highly ordered signal st ripes, whereas 4.1R protein(s) decorate the sarcoplasm in transverse striat ions that are in register with A-bands. An similar to 105/110-kDa 4.1R isof orm appears to occur in vivo in a supramolecular complex with major sarcome ric proteins, including myosin, alpha -actin, and alpha -tropomyosin. In vi tro binding assays showed that 4.1R may interact directly with the aforemen tioned contractile proteins through its 10-kDa domain. All of these observa tions suggest a topological model whereby 4.1R may play a scaffolding role by anchoring the actomyosin myofilaments and possibly modulating their disp lacements during contraction/relaxation.