Abbreviations used in this paper: FP, fl uorescent protein; SILAC, stable isotope labeling with amino acids in cell culture.The online version of this article contains supplemental material.

Significance We have developed a quantitative Förster resonance energy transfer-based methodology using multiphoton fluorescence lifetime imaging microscopy to investigate in single live cells the interaction between the substrate adaptor protein Kelch-like ECH associated protein 1 (Keap1) and transcription factor NF-E2 p45-related factor 2 (Nrf2), the master regulator of the mammalian cell response to oxidants and electrophiles. The application of this methodology revealed that the system is much more dynamic than previously anticipated, whereby Keap1 utilizes a unique cyclic mechanism to target Nrf2 for ubiquitination and proteasomal degradation, which also leads to Keap1 regeneration. The nature of this mechanism allows rapid transcriptional responses to environmental changes and is capable of accommodating multiple modes of regulation.

FRET analysis of cell lines expressing fluorescently tagged histones on separate nucleosomes demonstrates that variations in chromosome compaction occur during mitosis.