Inside the crowded environment of a cell nucleus where DNA is folded into a multi-layered three-dimensional chromatin network, biological proteins team up into dimers and high-order oligomers to refine their exploration volume via size exclusion and facilitate navigation of the nuclear landscape. Directly visualizing this process in the nucleoplasm of a living cell is, however, a complex task. Thus, to do just this, we established a fluorescence microscopy method that can extract the mobility of a fluorescently tagged nuclear protein as a function of oligomeric state, and then spatiotemporally map the anisotropy of this parameter with respect to nuclear architecture. This biophysical tool is based on radial pair correlation of molecular brightness (pCOMB) fluctuations, and it has the capacity to be an invaluable asset for cell biologists who want to investigate DNA target search. 

Along this line, the cover image of the June 7 issue of Biophysical Journal presents radial pCOMB maps of the diffusive route that a transcription factor called “signal transducer and activator of transcription 3” (blue lines) adopts with respect to a live-cell chromatin network during a dimer-to-tetramer transition (green and red lines). Importantly, although we focus on the cell nucleus in our study, radial pCOMB can be applied to any intracellular compartment. More information about our research and how fluorescence fluctuation spectroscopy can be used to uncover intracellular protein dynamics within live-cell architecture can be found at https://cellularbiophysics.science.unimelb.edu.au/ and https://www.lfd.uci.edu/.

- Ashleigh Solano, Jieqiong Lou, Lorenzo Scipioni, Enrico Gratton, and Elizabeth Hinde