Ultramarine, a Chromoprotein Acceptor for Forster Resonance Energy Transfer

We have engineered a monomeric blue non-fluorescent chromoprotein called Ultramarine (fluorescence quantum yield, 0.001; epsilon (585) (nm), 64,000 M-1. cm(-1)) for use as a Forster resonance energy transfer acceptor for a number of different donor fluorescent proteins. We show its use for monitoring activation of caspase 3 in live cells using fluorescence lifetime imaging. Ultramarine has the potential to increase the number of cellular parameters that can be imaged simultaneously.