Alzheimer's disease is characterized by two protein precipitates, extracellular amyloid plaques and intracellular neurofibrillary tangles (NFTs). The primary constituent of NFTs is a hyperphosphorylated form of the microtubule-binding protein tau. Hyperphosphorylation of tau on over 30 residues, primarily within proline-rich sequences, is associated with conformational changes whose nature is poorly defined. Peptides derived from the proline-rich region of tau (residues 174-242) were synthesized, and the conformations were analyzed for the nonphosphorylated and phosphorylated peptides. CD and NMR data indicate that phosphorylation of serine and threonine residues in proline-rich sequences induces a conformational change to a type II polyproline helix. The largest phosphorylation-dependent conformational changes observed by CD were for tau peptides incorporating residues 174-183 or residues 229-238. Phosphoserine and phosphothreonine residues exhibited ordered values of (3)J(alphaN) (3.1-6.2 Hz; mean = 4.7 Hz) compared to nonphosphorylated serine and threonine. Phosphorylation of a tau peptide consisting of tau residues 196-209 resulted in the disruption of a nascent alpha-helix. These results suggest that global reorganization of tau may occur upon hyperphosphorylation of proline-rich sequences in tau.