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

The thermal unfolding of monomeric Acanthamoeba myosin II and rabbit s keletal muscle myosin at pH 7.5 in 0.6 M KCl has been studied by diffe rential scanning calorimetry (DSC) and circular dichroism, A single en dotherm (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 obse rved for both dephospho- and phospho-myosin II, Skeletal muscle myosin unfolds with less cooperativity over a wider temperature range (simil ar to 40 to 60 degrees C) with Delta H approximate to 2500 kcal/mol, T he thermal unfolding of either myosin results in a loss of similar to 70% of a-helical structures, Saturation of dephospho- or phospho-myosi n 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 regi ons by nucleotide binding, Indeed, a purified N-terminal myosin II hea d fragment has been found to unfold with T-max similar to 41 and simil ar 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 sti ll smaller with ADP alone, In summary, thermally induced unfolding of myosin II is affected by nucleotide binding to heads, but not by phosp horylation or even removal of a 66-amino-acid tailpiece containing pho sphorylation sites, The observed differences in the cooperativity of u nfolding myosin II and skeletal muscle myosin relate to differences be tween rod structures and possibly also head-rod interactions.