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Acid‐induced dimerization of skeletal troponin C
Author(s) -
Wang ChienKao,
Lebowitz Jacob,
Cheung Herbert C.
Publication year - 1989
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.340060409
Subject(s) - chemistry , dimer , troponin c , skeletal muscle , troponin , monomer , troponin i , intramolecular force , crystallography , biophysics , stereochemistry , organic chemistry , anatomy , biology , psychology , psychiatry , myocardial infarction , polymer
We have investigated pH‐dependent changes of the properties of troponin C from rabbit skeletal muscle. At pH 7.5 this protein is a monomer and at pH 5.2 it is a dimer. In contrast, bovine cardiac troponin C remains essentially monomeric at pH 5.2. Bovine brain calmodulin is not a dimer, but significantly aggregated at the same acidic pH. The dimerization of skeletal troponin C was demonstrated by low‐speed (16,000 rpm) sedimentation equilibrium measurements carried out at 20°C and by polyacrylamide gel electrophoresis under nondenaturing conditions. Dimer formation was significantly inhibited in the ultracentrifuge at rotor speeds of 30,000 and 40,000 rpm at 20°C, and was completely prevented at a rotor speed of 40,000 rpm and 4°C. This temperature and pressure dependence of dimerization strongly suggests that hydrophobic bonding is a major factor in promoting skeletal troponin C association at pH 5.2. The intramolecular distance between Met‐25 and Cys‐98 of rabbit skeletal troponin C deduced from fluorescence resonance energy transfer measurements increased by a factor of two upon lowering the pH from 7.5 to 5.2, indicating a pH‐dependent transition in which the protein changed from a relatively compact conformation to an elongated conformation. The protein‐induced increase in the energy transfer distance is related to the acid‐induced dimerization of the protein. The extended conformation observed at pH 5.2 is compatible with the dumbbell‐shaped structure of skeletal troponin C crystals obtained from turkey at pH 5.0 [Herzberg, O., James, M. N. G. Nature (London) 313:653–659, 1985] and chicken at pH 5.1 (Sundaralingam, M., Bergstrome, R., Strasburg, G., Rao, S. T., Roychowdhury, P., Greaser, M., Wang, B. C. Science 227:945–948, 1985). However, the conformation in neural solution deviates form that predicted by crystallography. Intermolecular interactions leading to dimer formation likely play an important role in promoting the extended conformation that exists at acidic pH.