Research Grant

Sandra de Macedo Ribeiro, PhD

Instituto de Biologia Molecular e Celular, Porto, Portugal
Understanding the ubiquitin code in disease: From protein function to protein structure

Machado-Joseph disease (MJD) is a rare neurodegenerative disorder caused by expansion of a trinucleotide repeat in the affected gene. This expansion at the genetic level is translated into a polyglutamine expansion in the affected protein – ataxin-3 – which then forms aggregates that are toxic to neurons.

The cellular function of ataxin-3 is still not totally clear and the manner by which it is affected by the polyglutamine expansion is unknown, but there is evidence pointing to its involvement in the quality-control of protein folding and expression. Ataxin-3 functions as a deubiquitinating enzyme, with the ability to cleave specific polyubiquitin chains (signals added to proteins that direct them for degradation or to different locations in the cells). Most curiously, ataxin-3 is able to mitigate neurodegeneration induced by expanded polyglutamine proteins, a property that is dependent on its deubiquitinating activity.

Still, and despite intense research to determine the cellular role of this enzyme with singular preferences for different polyubiquitin chains, we still do not understand the mechanism of action of ataxin-3 nor how it is affected by expansion of the polyglutamine region in disease. The specific folding of a protein produces exposed regions which interact chemically and physically with similar regions in other proteins. The interactions of these exposed domains may be characterized by determining the three-dimensional shape of the interacting partners (isolated and/or together), by methods such as nuclear magnetic resonance and X-ray crystallography. Mutations, including polyglutamine expansions, may alter the structure of a protein and hence its interactions with other proteins through these exposed regions. The aim of this project is to generate reagents that allow us to probe how ataxin-3 recognizes its polyubiquitin partners, its mechanism of action and its three-dimensional shape.