Structural and thermodynamic characterization of an intrinsically disordered complex

CHEMES L.B.

Santiago de Chile

Conferencia; 3rd Protein Biophysics at the End of the World Symposium; 2018

Universidad Católica, Biophysical Society, ICGEB

The Retinoblastoma (pRb) tumor suppressor controls cell cycle progression throughout eukaryotes and is targeted by multiple viruses to alter the cell proliferative state, leading to oncogenic phenotypes. One of the major mechanisms evolved by viral proteins for disrupting host protein-protein interactions is the mimicry of short linear motifs (LMs). Intrinsic disorder is thought to promote high affinity interactions by increasing conformational flexibility and the accessibility of binding motifs. However, the role played by intrinsically disordered ?linker? regions in determining the binding strength of systems with multiple binding sites is still largely unknown.We use the Adenovirus E1A protein as a model system to study a multisite interaction. E1A uses two linear motifs joined by an disordered linker to bind pRb. Through a combination of biophysical measurements, NMR and modeling, we show that flexibility within the disordered linker allows for a striking enhancement of the affinity of individual motifs leading to the formation of a very tight complex. Random polymer chain models predict the compound affinity with striking accuracy, and suggest that the E1A linker behaves as a flexible polymer that allows optimal positioning of the motifs for binding to pRb. While NMR measurements confirm that the E1A linker remains highly flexible when bound to pRb, titrations reveal that several linker residues establish interactions with pRb, suggesting that the global entropic chain like behavior of the system is realized by an underlying compensation between repulsive interactions in the negatively charged linker and weak binding to pRb, which is evolutionarily conserved. This behavior is conserved throughout evolution, allowing the multiple interactions of viral IDPs.Our work provides quantitative understanding of the role played by disordered linkers in multi-site binding, and suggests that charge distribution, interactions and linker length are finely tuned throughout evolution to allow for effective competition with cellular interactions.