Phosphorylation by GSK-3β is sufficient to promote filament clustering. Panel A and B show products of phosphorylated and polymerized tau (+phos.). Panel C and D show products of phosphorylated tau that was polymerized in the presence of 20 μM ThS (+phos., +ThS). The clusters of filaments were approximately 2 μm across their longest axis. Magnification was 20,000 × (scale bar represents 500 nm). Panels A and C were induced with 25 μM ARA and diluted five fold prior to grid preparation; Panels B and D were induced with 75 μM ARA, and diluted 10 fold.