Role of tropomyosin isoforms in the calcium sensitivity of striated muscle thin filaments.
We have expressed alpha & beta isoforms of mammalian striated muscle tropomyosin (Tm) and alpha-Tm carrying the D175N or E180G cardiomyopathy mutations. In each case the Tm carries an Ala-Ser N-terminal extension to mimic the acetylation of the native Tm. We show that these Ala-Ser modified proteins are good analogues of the native Tm in the assays used here. We go on to use an in vitro kinetic approach to define the assembly of actin filaments with the Tm isoforms with either a cardiac or a skeletal muscle troponin (cTn, skTn). With skTn the calcium sensitivity of the actin filament is the same for alpha & beta-Tm and there is little change with the mutant Tms. For cTn switching from alpha to beta-Tm causes an increase of calcium sensitivity of 0.2 pCa units. D175N is very similar to the wild type alpha-Tm and E180G shows a small increase in calcium sensitivity of about 0.1 pCa unit. The formation of the switched-off blocked-state of the actin filament is independent of the Tm isoform but does differ for cardiac versus skeletal Tn. The in vitro assays developed here provide a novel, simple and efficient method for assaying the behaviour of expressed thin filament proteins.
Boussouf
Sabrina E
SE
Department of Biosciences, University of Kent, Canterbury CT2 7NJ, UK.
Maytum
Robin
R
Jaquet
Kornelia
K
Geeves
Michael A
MA
D008841:Actin Cytoskeleton / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D009202:Cardiomyopathies; D006801:Humans; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D011817:Rabbits; D051381:Rats; D016415:Sequence Alignment; D014335:Tropomyosin / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism*
D020033:Protein Isoforms; D014335:Tropomyosin; D014336:Troponin; D002118:Calcium