Phosphorylation of tau alters its association with the plasma membrane

1. The potential functions of the microtubule-associated protein tau have b een expanded by the recent demonstration of its interaction with the plasma membrane. Since the association of fan with microtubules is regulated by p hosphorylation, herein we examine whether or not the association of tau wit h the plasma membrane is also regulated by phosphorylation. 2. A range of tau isoforms migrating from 46 to 64 kDa was associated with crude particulate fractions derived from SH-SY-5Y human neuroblastoma cells , and were retained during the initial stages of plasma membrane purificati on. During the extensive washing utilized in purification of the plasma mem brane, portions of each of these isoforms were depleted from the resultant purified membrane. Immunoblot analysis with phospho-dependent and -independ ent antibodies revealed selective depletion of phospho isoforms during memb rane washing. This effect was more pronounced for the slowest-migrating (64 -kDa) tau isoform. 3. This putative influence of phosphorylation on the association of tau wit h the plasma membrane was further probed by transfection of SH-SY-5Y human neuroblastoma cells with a tau construct that could associate with the plas ma membrane but not with microtubules. Treatment with phorbol ester or calc ium ionophore, both of which increased phospho-tau levels within the cytoso l and plas:ma membrane, was accompanied by the dissociation of this tau con struct from the membrane. 4. These data indicate that phosphorylation regulates the association with the plasma membrane. Dissociation from the membrane by phosphorylation may place tau at risk for hyperphosphorylation and ultimate PHF formation in a manner previously considered for tau dissociated from microtubules.