Light Microscopy and Dynamic Light Scattering to Study Liquid-Liquid Phase Separation of Tau Proteins In Vitro

Tau is an intrinsically disordered protein that binds and stabilizes axonal microtubules (MTs) in neurons of the central nervous system. The binding of Tau to MTs is mediated by its repeat domain and flanking proline-rich domains. The positively charged (basic) C-terminal half of Tau also mediates the assembly Tau into fibrillar aggregates in Alzheimer's disease (AD) and tauopathy brains. In recent years, another assembly form of Tau has been identified: Tau can undergo liquid-liquid phase separation (LLPS), which leads to its condensation into liquid-dense phases, either by complex coacervation with polyanions like heparin or RNA or through "self-coacervation" at high Tau concentrations. Condensation of Tau in the absence of polyanions can be enhanced by the presence of molecular crowding agents in a dilute Tau solution. In vitro experiments using recombinant purified Tau are helpful to study the physicochemical determinants of Tau LLPS, which can then be extrapolated into the cellular context. Tau condensation is a new aspect of Tau biology that may play a role for the initiation of Tau aggregation, but also for its physiological function(s), for example, the binding to microtubules. Here we describe how to study the condensation of Tau in vitro using light microscopy, including fluorescence recovery after photobleaching (FRAP), to assess the shape and molecular diffusion in the condensates, a proxy for the degree of condensate percolation. We also describe turbidity measurements of condensate-containing solutions to assess the overall amount of LLPS and time-resolved dynamic light scattering (trDLS) to study the formation and size of Tau condensates.

Catalytic studies of glutathione transferase from Clarias gariepinus (Burchell) in dilute and crowded solutions

Kinetic properties of purified Clarias gariepinus glutathione transferase (CgGST) was studied in the presence of Ficoll 70, Polyethylene glycol (PEG) 6000, bovine serum albumin (BSA) and in dilute solution. This was done to mimic the cytosol thereby unraveling the actual mechanism of detoxication involving glutathione transferase (GST) in the crowded intracellular milieu. CgGST from the liver of Clarias gariepinus was purified to homogeneity by affinity chromatography on glutathione (GSH) - agarose. Initial-velocity study was performed by varying the concentrations of GSH at various fixed concentrations of 1-chloro-2,4-dinitrobenzene (CDNB) and vice-versa. Data obtained were fitted to the three equations representing random-ordered, compulsory-ordered and ping-pong mechanisms to obtain kinetic parameters. Product inhibition studies using sodium chloride (NaCl) was done by varying the concentrations of NaCl and CDNB at a fixed concentration of GSH and vice-versa. Data obtained were fitted to three equations representing competitive, non-competitive and uncompetitive inhibitions to obtain the inhibition constants (K_i^GSH and K_i^CDNB). Optimal temperature of CgGST activity was 20 °C both in dilute and crowded solutions. Maximum velocity (V_max) in dilute solution was decreased, while K_m^GSH and K_m^CDNB were increased in the presence of the crowding agents. Turnover number (k_cat), catalytic efficiency - k_cat/K_m^GSH_,k_cat/K_m^CDNB and inhibition constants - (K_i^GSH and K_i^CDNB) were reduced in crowded solutions. Mechanism of catalysis was steady - state random sequential in both dilute and crowded solutions. The study concluded that although the catalytic efficiency of the enzyme was reduced in crowded solution, mechanism of catalysis remains the same in both crowded and dilute solutions.

Quaternary structure of human small heat shock protein HSPB6 (Hsp20) in crowded media modeled by trimethylamine N-oxide (TMAO): Effect of protein phosphorylation

Effect of trimethylamine N-oxide (TMAO), well-known osmolyte, widely used to imitate crowded intracellular conditions, on the quaternary structure of recombinant human small heat shock protein HspB6 (Hsp20) was analyzed by means of size-exclusion chromatography, chemical crosslinking and analytical ultracentrifugation. Consistent with previous reports, in the absence of TMAO unphosphorylated, pseudophosphorylated (S16D mutant) and phosphorylated HspB6 form only small oligomers (presumably dimers). Addition of TMAO to unphosphorylated HspB6 leads to formation of different large oligomers being in equilibrium with dimers. Pseudophosphorylation (S16D mutation) or phosphorylation partially or completely prevent TMAO-induced oligomerization of HspB6. Pseudophosphorylation affects bis-ANS binding suggesting decreased hydrophobicity of HspB6. According to size-exclusion chromatography, TMAO-induced changes of HspB6 oligomerization result in its altered interaction with HspB1 and this effect can be reversed by HspB6 phosphorylation. It is concluded that under conditions of molecular crowding, characteristic for intracellular environment, HspB6 undergoes reversible changes of its oligomeric state which can affect its physiologically important properties and can be delicately regulated by phosphorylation.