Dynamic and quantitative Ca(2+) measurements using improved cameleons

Cameleons are genetically-encoded fluorescent indicators for Ca(2+) based on green fluorescent protein variants and calmodulin (CaM). Because cameleons can be targeted genetically and imaged by one- or two-photon excitation microscopy, they offer great promise for monitoring Ca(2+) in whole organisms, tissues, organelles, and submicroscopic environments in which measurements were previously impossible. However, the original cameleons suffered from significant pH interference, and their Ca(2+)-buffering and cross-reactivity with endogenous CaM signaling pathways was uncharacterized. We have now greatly reduced the pH-sensitivity of the cameleons by introducing mutations V68L and Q69K into the acceptor yellow green fluorescent protein. The resulting new cameleons permit Ca(2+) measurements despite significant cytosolic acidification. When Ca(2+) is elevated, the CaM and CaM-binding peptide fused together in a cameleon predominantly interact with each other rather than with free CaM and CaM-dependent enzymes. Therefore, if cameleons are overexpressed, the primary effect is likely to be the unavoidable increase in Ca(2+) buffering rather than specific perturbation of CaM-dependent signaling.