Linking structure – dynamics – function in intrinsically disordered regions of multidomain regulatory proteins

Intrinsically disordered regions (IDRs) of multidomain proteins lack a well-defined three-dimensional shape compared to globular regions. IDRs are very dynamic and commonly involved in crucial biological functions such as DNA replication, cell growth, and development. However, IDRs can be challenging for structural characterization using traditional approaches such as crystallography, NMR, or electron microscopy. Hence, the complete picture of the function of multidomain proteins is often hindered by a lack of structural knowledge about their IDRs. The overall aim of this work is to characterize the conformation of IDRs during their interactions with folded domains or other binding partners to expand our understanding of these important domains. To answer this, we use a combination of several biophysical techniques, such as small-angle X-ray/neutron scattering (SAXS/SANS), coupled with modeling to obtain important structural parameters that together sheds light on functional significance of IDRs. We studied two multidomain proteins with N-terminal IDRs. In the first case, we investigated how the N-terminal IDR interacts with and modulates the conformation of an adjacent folded domain of c-Src kinase, a multidomain oncoprotein. Secondly, we investigated the N-terminal IDR of companion of cellulose synthase 1 (CC1), a transmembrane protein that regulates cellulose synthesis under stress conditions. In both cases, we found a common mechanism of action for both proteins whereby they maintained their disordered conformation during interactions with their binding partners by forming weak multivalent interactions. Furthermore, in the case of CC1NTD, we investigated its propensity for self-association to form liquid – liquid phase separation that may have important implications for CC1 to act as a stress response protein. Overall, we can propose that the ability of these IDRs to facilitate multiple intra- and intermolecular weak interactions is key for their functional role. This work provides new fundamental information about the function of IDRs in multidomain proteins that may be important for different applications including therapeutic development and synthesis of novel self-associating biomaterials.