Isolated dystrophin molecules as seen by electron microscopy.

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Abstract
Dystrophin, the protein product of the Duchenne muscular dystrophy locus [Hoffman, E. P., Brown, R. H., Jr., & Kunkel, L. M. (1987) Cell 51, 919-928], is expressed in striated and smooth muscles as well as in non-muscle tissues. Examination of its primary structure has revealed that the molecule is composed of four domains, three of which share many features with the membrane cytoskeletal proteins spectrin and actinin. Dystrophin has thus been predicted to adopt a rod shape [Koenig, M., Monaco, A. P. & Kunkel, L. M. (1988) Cell 53, 219-228]. In the present study, we describe its isolation from the chicken gizzard smooth muscle and present electron microscopic images of the molecule. Polyclonal antibodies were first prepared from a dystrophin fragment derived from the chicken skeletal muscle gene (residues 1173-1728). A dystrophin-enriched membrane preparation from chicken gizzard muscle was then purified by passing it through an affinity chromatography column made with the anti-dystrophin antibodies. Electron microscopy of isolated and rotatory-shadowed dystrophin molecules revealed that the lengths measured for the dystrophin monomers (175 +/- 15 nm) are compatible with a structural arrangement of the repeat sequence segments in triple-barrel alpha-helices connected by short-turn regions, as was earlier postulated for the repeat domains of spectrin and actinin. Electron microscopic images indicate that in addition the dystrophin molecules could present the same capacity of self-association in oligomeric structures as these cytoskeletal proteins and may thus be a part of a complex molecular meshwork essential to muscle cell function.