Two species of alpha-actinin from rabbit fast skeletal muscles were identified with a monospecific antisera. Designated alpha-actinin1f and alpha-actinin2f, their distribution in muscles does not correlate with histochemically defined fast fiber type. Rather, the presence of each correlates with Z-line width and with the expression of different thin filament Ca2+-regulatory complexes. alpha-Actinin1f is expressed with troponin T 1f-alpha beta tropomyosin, and alpha-actinin2f with troponin T 2f-alpha 2 tropomyosin. CNBr peptide maps show that the fast alpha-actinin species differ in primary structure. In contrast, the slow alpha-actinin is indistinguishable from alpha-actinin1f. Further evidence for the similarity of alpha-actinin1f and slow alpha-actinin comes from electron microscopic studies which show that fibers that express these species exhibit thick Z-lines. So, unlike other contractile proteins, the multiple forms of alpha-actinin do not reflect the distinction between fast- and slow-twitch muscles.

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