The study investigates the effects of phosphorylation on the ability of tau protein to promote microtubule assembly. Tau, a family of closely related proteins, is known for its ability to copolymerize with tubulin and form microtubules. When tau is treated with alkaline phosphatase to remove phosphate residues, it undergoes a significant change in electrophoretic mobility, likely due to a conformational shift. Dephosphorylated tau promotes faster and more extensive microtubule polymerization, but the microtubules formed are indistinguishable from those formed by untreated tau. The study also found that partially purified microtubule protein contains a kinase that can rephosphorylate tau. The results suggest that tau exists in two distinct phosphorylation states, with the dephosphorylated state (state I) having a higher electrophoretic mobility and the phosphorylated state (state II) having a lower mobility. The phosphorylation state of tau significantly affects its ability to promote microtubule assembly, with state I tau promoting faster and more extensive polymerization. The study highlights the potential regulatory role of tau phosphorylation in microtubule dynamics and suggests that the function of tau may be more complex than simply promoting microtubule assembly.The study investigates the effects of phosphorylation on the ability of tau protein to promote microtubule assembly. Tau, a family of closely related proteins, is known for its ability to copolymerize with tubulin and form microtubules. When tau is treated with alkaline phosphatase to remove phosphate residues, it undergoes a significant change in electrophoretic mobility, likely due to a conformational shift. Dephosphorylated tau promotes faster and more extensive microtubule polymerization, but the microtubules formed are indistinguishable from those formed by untreated tau. The study also found that partially purified microtubule protein contains a kinase that can rephosphorylate tau. The results suggest that tau exists in two distinct phosphorylation states, with the dephosphorylated state (state I) having a higher electrophoretic mobility and the phosphorylated state (state II) having a lower mobility. The phosphorylation state of tau significantly affects its ability to promote microtubule assembly, with state I tau promoting faster and more extensive polymerization. The study highlights the potential regulatory role of tau phosphorylation in microtubule dynamics and suggests that the function of tau may be more complex than simply promoting microtubule assembly.