Destruction of key regulatory proteins by the ubiquitin/proteasome system is crucial for many regulatory processes in the cell, and this system has been proposed to have a role in a number of disorders, including cancer and neurodegenerative diseases. Despite the existence of animal models for a number of these diseases, such as Alzheimer's and Parkinson's diseases, in vivo data concerning the ubiquitin/proteasome system are lacking. To address this need, Dantuma and colleagues have developed a model for in vivo quantitative analysis of the degradative machinery by generating transgenic mice that carry a green fluorescent protein (GFP) reporter with a constitutively active degradation signal, according to work published in Nature Biotechnology.

The process of removing intracellular proteins involves a complex network of enzymes that link multiple N-terminally linked ubiquitin molecules to the protein substrate, which targets the substrate for unfolding and subsequent degradation by the proteasome. To dissect this process in vivo, Lindsten et al. selected a GFP-reporter fused to an N-terminally linked ubiquitin that serves as an acceptor for further ubiquitin molecules, thereby automatically delivering the GFP to the proteasome for degradation. The GFP fusion protein was tolerated at high concentration in mammalian cells. Proteasome inhibitors given to the transgenic animals expressing this reporter substrate resulted in substantial accumulation of GFP in multiple tissues, confirming the in vivo functionality of the reporter. Furthermore, analysis of the transgenic animals revealed that accumulation of the reporter was induced in primary neurons by an aberrant ubiquitin that is found in Alzheimer's disease.

The role of the ubiquitin/proteasome system in diverse disorders, as well as its identification as a possible therapeutic target, makes these GFP-transgenic animals an important tool for monitoring the status of the ubiquitin/proteasome system in physiological and pathological conditions.