Conformational Heterogeneity and Organelle-Like Liquid-Liquid Phase Separation of Intrinsically Disordered Proteins
Intense research in the past one and a half decade demonstrated that not all proteins function as folded structures. Intrinsically disordered proteins or regions (referred to collectively as IDPs hereafter) perform critical physiological functions, especially for the regulation of cellular processes in higher organisms. Sequence-dependent conformational ensembles of intrinsically disordered proteins (IDPs) are heterogeneous, unlike those of a homopolymer. Conformational properties of individual IDP molecules have been characterized by NMR, SAXS, and smFRET. Applying explicit-chain simulations to experimental smFRET data on the cyclin-dependent kinase (CDK) inhibitor Sic1 as an exemplifying case, we developed a general subensemble method of smFRET inference of conformational dimensions of disordered proteins that does not presume a homogeneous ensemble. In contrast to conventional procedures that led to unphysically large smFRET-inferred radius of gyration (R_g) at high [GdmCl], the Sic1 compactness inferred by our method is in good agreement with SAXS data for R_g and NMR measurement of hydrodynamic radius. Remarkably, some IDPs function not only as individual molecules, but also act collectively by undergoing reversible liquid-liquid phase separation in the living cell. The resulting high-IDP phase forms a major component of membraneless organelles such as P granules and nucleolus that, by creating their own IDP-rich compartments, stimulate critical biological functions. To gain physical insight into these newly discovered and fascinating phenomena, I will discuss our recent effort in using analytical theory to elucidate how biologically functional phase separation of IDPs is governed by their amino acid sequences.