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Science 17 November 1989:
Vol. 246. no. 4932, pp. 926 - 928
DOI: 10.1126/science.2814515
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Articles

Science, Vol 246, Issue 4932, 926-928
Copyright © 1989 by American Association for the Advancement of Science

articles

Assembly of the native heterodimer of Rana esculenta tropomyosin by chain exchange

SS Lehrer, YD Qian, and S Hvidt

Department of Muscle Research, Boston Biomedical Research Institute, MA 02114.

Rana esculenta tropomyosin assembles in vivo into a coiled-coil alpha helix from two different subunits, alpha and beta, which are present in about equal concentrations. Although the native composition is alpha beta, a mixture of equal amounts of alpha alpha and beta beta is produced by refolding dissociated alpha and beta at low temperature in vitro. Refolding kinetics showed that alpha alpha formed first and was relatively stable with regard to chain exchange below approximately 20 degrees C. Equilibration of the homodimer mixture at 30 degrees and 34 degrees C for long times, however, resulted in the formation of the native alpha beta molecule by chain exchange. Biosynthesis of alpha beta from separate alpha and beta genes is, therefore, favored thermodynamically over the formation of homodimers, and biological factors need not be invoked to explain the preferred native alpha beta composition.


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