The folding and binding mechanisms of the PTB domain of FRS2: an unexpected role of a disulfide bridge in rewiring folding and function

Protein-protein interactions are fundamental to cellular function and regulation, driving essential biological processes such as signaling, enzyme activity, and structural support. These interactions enable proteins to collaborate within signaling networks, where they often bind to one another with high specificity to mediate processes like signal transduction, cellular response to stimuli, and structural assembly. Proper protein folding is critical to ensuring that proteins attain their functional three-dimensional conformations, allowing them to effectively engage in these interactions. Disruptions in either protein folding or protein-protein binding can lead to diseases, highlighting the importance of these processes for cellular health and homeostasis. This PhD thesis focuses on the folding and binding dynamics of the PTB domain of the FRS2 protein, an adaptor protein that plays a pivotal role in cellular signaling by interacting with tyrosine kinase receptors. The PTB domain mediates these interactions through binding with both phosphorylated and unphosphorylated ligands, such as TrkB and FGFR1. We investigate the molecular mechanisms that govern both the folding of the PTB domain and its binding interactions, with particular emphasis on the role of the disulfide bond between cysteines C61 and C80 in regulating both processes. Our results demonstrate that the disulfide bond not only contributes to the overall stability of the protein, but its presence or absence induces a subtle conformational change that modulates ligand binding. This mechanism highlights that protein-protein interaction domains are not merely passive mediators that simply recognize ligands, but are subject to fine-tuned regulation. This regulation can be exemplified by the presence of the disulfide bond, strategically positioned far from the binding pocket, yet capable of modulating ligand recognition and interaction.