An ultramarine fluorescent protein with increased photostability and pH insensitivity.

We report a pH-insensitive and photostable ultramarine fluorescent protein, Sirius, with an emission peak at 424 nm, the shortest emission wavelength among fluorescent proteins reported to date. The pH-insensitivity of Sirius allowed prolonged visualization of biological events in an acidic environment. Two fluorescence resonance energy transfer (FRET) pairs, Sirius-mseCFP and Sapphire-DsRed, allowed dual-FRET imaging with single-wavelength excitation, enabling detection of Ca(2+) concentration and caspase-3 activation in the same apoptotic cells.