Recently, a 60/63 kDa cytoskeletal protein, highly homologous to the glycolytic enzyme phosphoglucomutase (PGM 1), was isolated from smooth muscle tissue and shown to localize in various adherens-type junctions of muscle and some nonmuscle cells. Since this protein, tentatively named 'aciculin', was enriched in muscle tissues and cells, we have attempted to study its expression and localization during myodifferentiation. C2C12 mouse myoblasts did not express any aciculin before cell fusion in culture. Immediately after cell fusion aciculin became detectable and its content continued to rise during myotube maturation. In early myotubes aciculin appeared first at cell tips and was predominantly localized to focal adhesions of immature myotubes. As myotubes matured in culture, aciculin became associated with growing myofibrils, and finally was found redistributed in striations, corresponding to sarcomere Z-discs. Immunoblotting showed that aciculin content in chicken breast skeletal muscle remained very low until day 11 of embryogenesis, but significantly increased in late prenatal and early postnatal development. By immunofluorescence, aciculin was not revealed in thigh skeletal muscle of day 11 chicken embryos, but was prominently localized at myotendinous junctions in thigh muscle of day 16 embryos. Myotendinous junctions appeared to be major sites of aciculin accumulation in developing and mature skeletal muscle fibers in vivo, suggesting some role for this protein in thin filament-membrane interactions and, potentially, in force transmission at these cell-matrix contacts. In adult skeletal muscle faint aciculin staining appeared at the sarcolemma and as striations in register with Z-discs. Since the protein was not identified in glycerinated myofibrils but was localized to striations in C2C12 myotubes and within the limited areas on skeletal muscle tissue sections, we conclude that aciculin is a component of skeletal muscle costameres. In cultured C2C12 myotubes we found some codistribution of aciculin with clusters of acetylcholine receptors, suggesting its presence at neuromuscular junctions. However, we did not detect any significant concentration of aciculin at neuromuscular junctions in both embryonic and adult skeletal muscle. Taken together, our data show that aciculin expression in skeletal muscle is differentiation-dependent and upregulated during muscle development, and that this novel cytoskeletal protein is a component of various cell-matrix adherens junctions in muscle cells.