Effects of nucleotide binding on thermal transitions and domain structure of myosin subfragment 1

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Abstract
The thermal unfolding and domain structure of myosin subfragment i (S1) from rabbit skeletal muscles and their changes induced by nucleotide binding were studied by differential scanning calorimetry. The binding of ADP to S1 practically does not influence the position of the thermal transition (maximum at 47.2°C), while the binding of the non‐hydrolysable analogue of ATP, adenosine 5′‐[β,γ‐imido]triphosphate (AdoPP[NH]P) to S1, or trapping of ADP in S1 by orthovanadate (Vi), shift the maximum of the heat adsorption curve for S1 up to 53.2 and 56.1°C, respectively. Such an increase of S1 thermostability in the complexes S1‐AdoPP[NH]P and S1‐ADP‐Vi is confirmed by results of turbidity and tryptophan fluorescence measurements. The total heat adsorption curves for S1 and its complexes with nucleotides were decomposed into elementary peaks corresponding to the melting of structural domains in the S1 molecule. Quantitative analysis of the data shows that the domain structure of S1 in the complexes S1‐AdoPP[NH]P and S1‐ADP‐Vi is similar and differs radically from that of nucleotide‐free S1 and S1 in the S1‐ADP complex. These data are the first direct evidence that the S1 molecule can be in two main conformations which may correspond to different states during the ATP hydrolysis: one of them corresponds to nucleotide‐free S1 and to the complex S1‐ADP, and the other corresponds to the intermediate complexes S1‐ATP and S1‐ADP‐Pi. Surprisingly it turned out that the domain structure of S1 with ADP trapped by p‐phenylene‐N, N′‐dimaleimide (p PDM) thiol cross‐linking almost does not differ from that of the nucleotide‐free S1. This means that pPDM‐cross‐linked S1 in contrast to S1‐AdoPP[NH]P and S1‐ADP‐Vi can not be considered a structural analogue of the intermediate complexes S1‐ATP and S1‐ADP‐Pi.