Abstract
The alpha and beta isoforms of tropomyosin (39 amino acid substitutions) were isolated from adult rabbit skeletal muscle without chemical modification. The levels of phosphorylation as judged by 2D PAGE were ∼10% (alpha) and nil (beta). Solutions of homodimeric beta tropomyosin (pH 7; ionic strength, ∼30 mM) display greater viscosity than those of alpha. Conversely, the enhancement of viscosity of the alpha isoform by skeletal troponin is greater than for beta. Mixtures of tropomyosin and the amino-terminal CNBr fragment of troponin-T, CB1 (res 1 - 151), were chromatographed on a size-exclusion column in the presence of different concentrations of KCl. In 0.1M salt, CB1 complexes and co-elutes with each isoform but is dissociated from either in 0.22 M. At intermediate salt, different degrees of complexation are observed - more extensive with alpha than beta. Further, the alpha-isoform elutes later in the salt gradient than the beta isoform during chromatography on skeletal troponin-Sepharose 4B. Tropomyosin isotype also has a bearing on thin filament regulation of the steady-state rate of actomyosin-S1 MgATPase. Reconstituted skeletal thin filaments composed solely of beta produce ∼30% less Ca(II)-induced activation compared to only alpha. This is observed at a high S1/actin ratio (6 uM myosin-S1A1, 3 uM thin filaments, pCa 4) and as a function of pCa (0.3 uM myosin-S1A1, 25 uM thin filaments).
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