Protein nanocondensates: the next frontier

Abstract

Over the past decade, myriads of studies have highlighted the central role of protein condensation in subcellular compartmentalization and spatiotemporal organization of biological processes. Conceptually, protein condensation stands at the highest level in protein structure hierarchy, accounting for the assembly of bodies ranging from thousands to billions of molecules and for densities ranging from dense liquids to solid materials. In size, protein condensates range from nanocondensates of hundreds of nanometers (mesoscopic clusters) to phase-separated micron-sized condensates. In this review, we focus on protein nanocondensation, a process that can occur in subsaturated solutions and can nucleate dense liquid phases, crystals, amorphous aggregates, and fibers. We discuss the nanocondensation of proteins in the light of general physical principles and examine the biophysical properties of several outstanding examples of nanocondensation. We conclude that protein nanocondensation cannot be fully explained by the conceptual framework of micron-scale biomolecular condensation. The evolution of nanocondensates through changes in density and order is currently under intense investigation, and this should lead to the development of a general theoretical framework, capable of encompassing the full range of sizes and densities found in protein condensates.

Keywords: Biomolecular condensates; Intrinsically disordered proteins; Membraneless organelles; Mesoscopic clusters, Nanocondensates, Protein coacervates; Phase separation; Protein colloids; Protein condensates; Protein conformation; Protein folding.

Fig. 1

Schematic protein-water phase diagram in the temperature–concentration plane. Below the solubility curve, supersaturated solutions are metastable and undergo transitions to the condensed liquid phase or to solid-like phases, such as gel states, crystalline states, amorphous solids, and fibers. The liquid-liquid binodal (coexistence curve) indicates the metastable boundary that maps the transition to the two-phase regime, where a liquid condensed phase coexists with a liquid dilute phase. The critical temperature (T_C) is the temperature at which the concentration differences between the two liquid phases vanishes and a homogeneous solution exists. At each temperature below T_C, the binodal defines pairs of protein concentrations, Φ_L and Φ_D, that characterize the protein volume fractions of the light and dense phases, respectively. The spinodal curve maps the boundary of the metastable regime. Below it, phase separation is abrupt and not limited by a nucleation energy barrier. The concentrations of the light and dense phases remain constant for a given temperature and are independent of the total protein concentration of the system, whereas the relative volumes of each phase change with the total concentration of the system. Thus, the left arm of the binodal maps the volume predominance of the light phase, whereas the volume of the dense phase predominates at the right arm. This explains why at low total concentrations, the system exhibits droplets of highly concentrated protein dispersed in a dilute phase, whereas at high total protein concentration, droplets of dilute protein are dispersed in a dense phase. The gelation line represents the protein concentration of gel phases. The solidus line defines the protein concentration of crystals, amorphous aggregates, and fibers