Present and Future Opportunities in Imaging the Ubiquitin System (Ub-System)

From yeast to mammalian cells, ubiquitination is one of the most conserved, and reversible, eukaryotic post-translational modifications (PTMs) responsible for controlling nearly all cellular processes. Potentially, every single eukaryotic cell can accomplish different ubiquitination processes at once, which in turn control the execution of specific cellular events in time and space with different biological significance (e.g., protein degradation or protein–protein interaction). Overall, all these signals are highly dynamic and need to be finely integrated to achieve a proper cellular response. Altogether, ubiquitination appears to be an extremely complex process, likely more than any other PTMs. Until a few years ago, the prevailing experimental approaches to investigate the different aspects of the ubiquitin system entailed genetic and biochemical analysis. However, recently, reagents and technologies have been developed enabling microscopy-based imaging of ubiquitination to enter the scene. In this paper, we discuss the progress made with conventional (confocal fluorescence microscopy) and non-conventional non-linear microscopy (Atomic Force Microscopy—AFM, Coherent Anti-Stokes Raman Scattering—CARS, Stimulated Raman Scattering—SRS) and we speculate on future developments.