Uncovering the early stages of domain melting in calmodulin with ultrafast temperature-jump infrared spectroscopy

Lucy Minnes, Gregory M. Greetham, Daniel J. Shaw, Ian P. Clark, Robby Fritzsch, Michael Towrie, Anthony W. Parker, Alistair J. Henry, Richard J. Taylor, Neil T. Hunt

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Abstract

The signaling protein calmodulin (CaM) undergoes a well-known change in secondary structure upon binding Ca2+, but the structural plasticity of the Ca2+-free apo state is linked to CaM functionality. Variable temperature studies of apo-CaM indicate two structural transitions at 46 and 58 °C that are assigned to melting of the C- and N-terminal domains, respectively, but the molecular mechanism of domain unfolding is unknown. We report temperature-jump time-resolved infrared (IR) spectroscopy experiments designed to target the first steps in the C-terminal domain melting transition of human apo-CaM. A comparison of the nonequilibrium relaxation of apo-CaM with the more thermodynamically stable holo-CaM, with 4 equiv of Ca2+ bound, shows that domain melting of apo-CaM begins on microsecond time scales with α-helix destabilization. These observations enable the assignment of previously reported dynamics of CaM on hundreds of microsecond time scales to thermally activated melting, producing a complete mechanism for thermal unfolding of CaM.

Original languageEnglish
Pages (from-to)8733-8739
Number of pages7
JournalJournal of Physical Chemistry B
Volume123
Issue number41
Early online date26 Sep 2019
DOIs
Publication statusPublished - 17 Oct 2019

Keywords

  • calmodulin
  • CaM
  • thermal melting

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