Localizable and Highly Sensitive Calcium Indicator Based on a BODIPY Fluorophore

We introduce here a new class of BODIPY-based Ca(2+) indicators which can be derivatized with biological ligands that permit the localization of the indicators in living cells. The underivatized BODIPY-based Ca(2+) indicator (BOCA-1) shows a 250-fold increase in fluorescence intensity upon Ca(2+) binding. We also prepared its O(6)-benzylguanine (BG) derivative, BOCA-1-BG, which can be covalently and specifically linked to SNAP-tag fusion proteins in living cells. The indicator retains its properties as a highly sensitive Ca(2+) indicator after conjugation to proteins, displaying a 180-fold increase in fluorescence intensity upon Ca(2+) binding. We further demonstrated that BOCA-1-BG through reaction with localized SNAP-tag fusion proteins can be used to sense changes in Ca(2+) concentrations in the nuclei and in the cytosol of live CHO-K1 cells. The high sensitivity of the indicator together with the possibility to selectively couple it to proteins of interest makes it a powerful tool for measuring local changes in Ca(2+) concentrations in living cells.