UC Santa Barbara

Microtubules (MTs) are protein nanotubes comprised of αβ-tubulin heterodimers that actas structural components of the cytoskeleton and carry a net negative charge. Tau, an intrinsically disordered neuronal protein and polyampholyte with an overall positive charge, is a microtubule associated protein, which binds to anionic domains of microtubules (MTs) and suppresses their dynamic instability. Aberrant tau-MT interactions are implicated in Alzheimer’s and other neurodegenerative diseases. Here, we studied the interactions between full length human protein tau and other negatively charged binding substrates, as revealed by differential-interference-contrast (DIC) and fluorescence microscopy. As binding substrates we chose anionic liposomes (ALs) containing either 1,2-dioleoyl-sn-glycero-3- phosphatidylserine (DOPS, -1e) or 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol (DOPG, - 1e) mixed with zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) to mimic anionic plasma membranes of axons where tau resides. At low salt concentrations (0 to 10 mM KCl or NaCl) with minimal charge screening, reaction mixtures of tau and ALs resulted in the formation of distinct states of AL-tau complexes coexisting with liquid-liquid phase separated tau self-coacervates arising from the polyampholytic nature of tau containing cationic and anionic domains. AL-tau complexes exhibited distinct types of morphologies, including, large ≈20-30 micron giant multi-lamellar liposomes with bound tau-membrane domains and finite-sized assemblies of smaller liposomes glued together through the cationic domains of tau. As the ionic strength of the solution was increased to near and above physiological salt concentrations (≈150 mM 1:1 electrolyte), AL-tau complexes remained stable while tau self-coacervate droplets were found to dissolve indicative of breaking of inter- and intra- tau (anionic/cationic) electrostatic bonds due to increased charge screening. The findings are consistent with the hypothesis that cationic domains of tau may interact with anionic domains of the lumen facing lipid monolayer of the axon plasma membranes (where most anionic lipids reside), suggesting the possibility of transient yet robust interactions at physiologically relevant ionic strengths.