THERMALLY-INDUCED UNFOLDING OF ACANTHAMOEBA MYOSIN-II AND SKELETAL-MUSCLE MYOSIN - NUCLEOTIDE EFFECTS

The thermal unfolding of monomeric Acanthamoeba myosin II and rabbit skeletal muscle myosin at pH 7.5 in 0.6 M KCl has been studied by differential scanning calorimetry (DSC) and circular dichroism, A single endotherm (at similar to 40 to 45 degrees C) with a maximum at 41.7 +/- 0.1 degrees C and Delta H approximate to 1080 +/- 180 kcal/mol is observed for both dephospho- and phospho-myosin II, Skeletal muscle myosin unfolds with less cooperativity over a wider temperature range (similar to 40 to 60 degrees C) with Delta H approximate to 2500 kcal/mol, The thermal unfolding of either myosin results in a loss of similar to 70% of a-helical structures, Saturation of dephospho- or phospho-myosin II with 5'-adenylylimidodiphosphate (AMPPNP) in the presence of Mg2+ produces a second endotherm with a maximum at similar to 49 degrees C, The latter observation is attributed to a stabilization of head regions by nucleotide binding, Indeed, a purified N-terminal myosin II head fragment has been found to unfold with T-max similar to 41 and similar to 48 degrees C in the absence and presence of AMPPNP, respectively, The stabilization of the head regions is less with ADP + P-i and still smaller with ADP alone, In summary, thermally induced unfolding of myosin II is affected by nucleotide binding to heads, but not by phosphorylation or even removal of a 66-amino-acid tailpiece containing phosphorylation sites, The observed differences in the cooperativity of unfolding myosin II and skeletal muscle myosin relate to differences between rod structures and possibly also head-rod interactions.