Calorimetric Studies of the Thermal Unfolding of Smooth Muscle Myosin Fragments and Their Complexes with ADP and Phosphate Analogs

D. A. Pavlov¹, A. Sobieszek², and D. I. Levitsky^1,3*

¹Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: dpavlov@pcman.genebee.msu.su

²Institute of Molecular Biology, Austrian Academy of Sciences, Billrothstrasse 11, A-5020 Salzburg, Austria; fax: +43 6245 828 467; E-mail: asobieszek@compuserve.com

³Bakh Institute of Biochemistry, Russian Academy of Sciences, Leninskii pr. 33, Moscow, 117071 Russia; fax: (095) 954-2732; E-mail: inbio@glas.apc.org

^* To whom correspondence should be addressed.

Received March 5, 1998; Revision received March 23, 1998

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The thermal unfolding of turkey gizzard smooth muscle myosin subfragment 1 (S1) and heavy meromyosin (HMM) in the absence of added nucleotides, in the presence of ADP, and in S1 or HMM ternary complexes with ADP and P_i analogs, orthovanadate (V_i), beryllium fluoride (BeF_x), or aluminum fluoride (AlF₄^-), have been studied by differential scanning calorimetry (DSC). It has been shown that the formation of these ternary complexes causes significant structural changes in S1 or in the heads of HMM which are reflected in a pronounced increase of the protein thermal stability. The effect of BeF_x was less distinct than that of AlF₄^- or V_i. Phosphorylation of regulatory light chains (RLC) in S1 or in HMM had practically no influence on these effects. In general, the changes caused by various P_i analogs in smooth muscle S1 or HMM were similar to those observed earlier with skeletal muscle S1 devoid of RLC. It is concluded that RLC and their phosphorylation do not significantly affect the character of structural changes induced in motor domains of the HMM heads by the formation of ternary complexes HMM--ADP--V_i, HMM--ADP--AlF₄^-, and HMM--ADP--BeF_x--stable analogs of the intermediate states of the HMM ATPase reaction, HMM·ADP·P_i and HMM·ATP.

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KEY WORDS: differential scanning calorimetry, myosin, subfragment 1, subfragment 2, heavy meromyosin, light chains, phosphorylation, smooth muscle

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