Deciphering the molecular mechanism responsible for GCaMP6m's Ca2+-dependent change in fluorescence

PLoS One. 2017 Feb 9;12(2):e0170934. doi: 10.1371/journal.pone.0170934. eCollection 2017.


The goal of this work is to determine how GCaMP6m's fluorescence is altered in response to Ca2+-binding. Our detailed spectroscopic study reveals the simplest explanation for how GCaMP6m changes fluorescence in response to Ca2+ is with a four-state model, in which a Ca2+-dependent change of the chromophore protonation state, due to a shift in pKa, is the predominant factor. The pKa shift is quantitatively explained by a change in electrostatic potential around the chromophore due to the conformational changes that occur in the protein when calmodulin binds Ca2+ and interacts with the M13 peptide. The absolute pKa values for the Ca2+-free and Ca2+-saturated states of GCaMP6m are critical to its high signal-to-noise ratio. This mechanism has important implications for further improvements to GCaMP6m and potentially for other similarly designed biosensors.

MeSH terms

  • Calcium / metabolism*
  • Fluorescence
  • Green Fluorescent Proteins / chemistry
  • Green Fluorescent Proteins / genetics
  • Green Fluorescent Proteins / metabolism*
  • HEK293 Cells
  • Humans
  • Molecular Dynamics Simulation
  • Protein Binding
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism


  • Recombinant Proteins
  • Green Fluorescent Proteins
  • Calcium