Key result
At low ionic strength, the aggregation of myosin subfragment 1 containing light chain A1 was strongly dependent on protein concentration and occurred at lower temperatures than the A2 isoform.
The N-terminal extension of the A1 essential light chain mediates intermolecular interactions that affect the aggregation properties of the myosin head at low ionic strength, which may reflect intramolecular interactions relevant to muscle contraction.
Findings remain preliminary in animal models; leaves open whether A1-mediated myosin interactions influence human cardiac contractility.
We compared the thermal aggregation properties of two isoforms of the isolated myosin head (myosin subfragment 1, S1) containing different "essential" (or "alkali") light chains, A1 or A2. Temperature dependencies for the aggregation of these two S1 isoforms, as measured by the increase in turbidity, were compared with the temperature dependencies of their thermal denaturation obtained from differential scanning calorimetry (DSC) experiments. At relatively high ionic strength (in the presence of 100 mM KCl) close to its physiological values in muscle fibers, we have found no appreciable difference between the two S1 isoforms in their thermally induced aggregation. Under these conditions, the aggregation of both S1 isoforms was independent of the protein concentration and resulted from their irreversible denaturation, which led to the cohesion of denatured S1 molecules. In contrast, a significant difference between these S1 isoforms was revealed in their aggregation measured at low ionic strength. Under these conditions, the aggregation of S1 containing a light chain A1 (but not A2) was strongly dependent on protein concentration, the increase of which (from 0.125 to 2.0 mg/ml) shifted the aggregation curve by ~10 degrees towards the lower temperatures. It has been concluded that the aggregation properties of this S1 isoform at low ionic strength is basically determined by intermolecular interactions of the N-terminal extension of the A1 light chain (which is absent in the A2 light chain) with other S1 molecules. These interactions seem to be independent of the S1 thermal denaturation, and they may take place even at low temperature.
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Markov et al. (2010) studied this question. Myosin essential light chain A1 vs. Myosin essential light chain A2 was evaluated on Thermal aggregation properties (temperature of half-maximum increase in optical density). At low ionic strength, the aggregation of myosin subfragment 1 containing light chain A1 was strongly dependent on protein concentration and occurred at lower temperatures than the A2 isoform.
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