Exploring molecular determinants of charged disordered protein interactions, phase separation and function from the test-tube to the cell
Biological molecular recognition is centred around the structure-function paradigm, involving interfacial complementary shapes and non-covalent forces. Intrinsically disordered proteins (IDPs), constituting up to 40% of the proteome, evade the structure-function paradigm, owing to their lack of persistent structure. Despite some IDPs forming folded complexes, numerous IDPs form disordered complexes stabilized by multivalent interactions. An extreme example is the recently discovered disordered high-affinity complex formed by two highly charged IDPs, the nucleosome binding linker histone (H1) and its nuclear chaperone prothymosin-alpha (ProTa), both key regulators of chromatin-structure and -function. IDPs are also often implicated in liquid-liquid phase separation (LLPS) driven assemblies that facilitate numerous biochemical interactions. The specificity question in IDPs becomes particularly pertinent for such charge driven disordered interactions and associated LLPS processes considering the great abundance of such highly charged disordered stretches in our proteome. Specific physiological function of most IDP interactions and associated LLPS processes imply an important role of specificity, albeit encoded differently from folded proteins. Only a multidisciplinary study spanning several complexity regimes, which is currently lacking, can uncover such altered specificity. I hypothesize that physiologically, ProTa/H1 interaction plays out in LLPS-driven heterochromatin assemblies leading to transcriptional regulation. I will probe ProTa/H1 interactions and phase behaviour with a set of systematically designed variants in incrementally complex milieu, from the test-tube to the cell, to reveal how the sequence dictates interactions and phase behaviour in-vitro and attribute a given function in-cell. This will shed light on specificity determinants of interaction and function for such highly charged IDPs.