Summary
One of the basic principle of living organisms is to separate the interior from the surrounding environment by a barrier. At the level of cells, this barrier is formed by a lipid bilayer called plasma membrane that separates the cytosol (interior) from the extracellular environment. In eukaryotic cells, the cytosol is organised between specialised compartments, which function in the distribution of proteins, nutrients and metabolites, assembly and degradation of macromolecules and export materials outside the cell. To fulfil these functions, transport and exchange between different compartments are essential. One important transport pathway is the secretory pathway, which allows the transport of lipids, proteins and other cargo molecules to the plasma membrane where export takes place. Most of secreted proteins use a conventional pathway that was believed to be the only route for secretion. However, this view has been recently challenged and it is now clear that alternative routes are used by several proteins to reach the cell surface. The original evidence for such unconventional secretory pathway came from the inhibition of the conventional pathway that did not inhibit the transport of several proteins to the cell surface. The characterisation of this unconventional transport pathway is very limited and represents a major gap in our understanding of protein trafficking. My project aims at gaining an insight into the mechanism of this unconventional protein transport using cutting-edge technologies. This is an important area of research, not only through its fundamental aspect, but also through its implication in human health as this alternative secretion system has been associated with several diseases such as cancer, infection, and metabolic and autoimmune disorders.
Technical summary
Most eukaryotic proteins are secreted following the conventional endoplasmic reticulum (ER)/Golgi secretory pathway. However, several cytosolic proteins lacking signal peptide for secretion (leaderless proteins) were shown to reach the cell surface by non-conventional transport pathways. Such proteins include members of the Annexin family, glycolytic enzymes, cytoskeleton proteins, Heat Shock Proteins, interleukins, fibroblast growth factors, β-galactoside-specific lectins and transglutaminases. In the extracellular environment, these macromolecules were shown to play a crucial role not only in normal physiology but also in human diseases (cancer, viral infection, autoimmune and metabolic disorders). Although function of leaderless proteins at the cell surface is relatively well documented and hundreds of papers have been published on this topic, the nature of the translocation mechanism has not been described yet and represents an important gap in our understanding of protein trafficking. My project aims at gaining an insight into the mechanism of this unconventional protein transport using biochemical, biophysical and cell biology approaches. My strategy turns around two axes. Firstly a hypothesis-driven approach consisting of testing and revisiting different models previously proposed in the literature. Secondly a data-driven approach identifying new regulators of this unconventional protein transport using genome-scale functional screens based on CRISPR/Cas9 technology. Both approaches, iteratively feeding on each other, should significantly enhance our understanding of what is currently a set of intriguing observations in an unbiased way.
