DEVELOPMENTAL EXPRESSION OF SPECTRINS IN RAT SKELETAL-MUSCLE

Skeletal muscle contains spectrin (or spectrin I) and fodrin (or spect rin II), members of the spectrin supergene family. We used isoform-spe cific antibodies and cDNA probes to investigate the molecular forms, d evelopmental expression, and subcellular localization of the spectrins in skeletal muscle of the rat. We report that beta-spectrin (beta I) replaces beta-fodrin (beta II) at the sarcolemma as skeletal muscle fi bers develop. As a result, adult muscle fibers contain only alpha-fodr in (alpha II) and the muscle isoform of beta-spectrin (beta I Sigma 2) . By contrast, other types of cells present in skeletal muscle tissue, including blood vessels and nerves, contain only alpha- and beta-fodr in. During late embryogenesis and early postnatal development, skeleta l muscle fibers contain a previously unknown form of spectrin complex, consisting of alpha-fodrin, beta-fodrin, and the muscle isoform of be ta-spectrin. These complexes associate with the sarcolemma to form lin ear membrane skeletal structures that otherwise resemble the structure s found in the adult. Our results suggest that the spectrin-based memb rane skeleton of muscle fibers can exist in three distinct states duri ng development.