Role of phosphorylation in the conformation of tau peptides implicated in Alzheimer's disease

A series of peptides corresponding to isolated regions of Tau (tau) protein have been synthesized and their conformations determined by H-1 NMR spectr oscopy. Immunodominant peptides corresponding to tau(224-240) and a bisphos phorylated derivative in which a single Thr and a single Ser are phosphoryl ated at positions 231 and 235 respectively, and which are recognized by an Alzheimer's disease-specific monoclonal antibody, were the main focus of th e study. The nonphosphorylated peptide adopts essentially a random coil con formation in aqueous solution, but becomes slightly more ordered into P-typ e structure as the hydrophobicity of the solvent is increased by adding up to 50% trifluoroethanol (TFE). Similar trends are observed for the bisphosp horylated peptide, with a somewhat stronger tendency to form an extended st ructure, There is tentative NMR evidence for a small population of species containing a turn at residues 229-231 in the phosphorylated peptide, and th is is strongly supported by CD spectroscopy. A proposal that the selection of a bioactive conformation from a disordered solution ensemble may be an i mportant step (in either tubulin binding or in the formation of PHF) is sup ported by kinetic data on Pro isomerization. A recent study showed that Thr 231 phosphorylation affected the rate of prolyl isomerization and abolished tubulin binding. This binding was restored by the action of the prolyl iso merase Pin1. In the current study, we find evidence for the existence of bo th trans and cis forms of tau peptides in solution but no difference in the equilibrium distribution of cis-trans isomers upon phosphorylation. Increa sing hydrophobicity decreases the prevalence of cis forms and increases the major trans conformation of each of the prolines present in these molecule s. We also synthesized mutant peptides containing Tyr substitutions precedi ng the Pro residues and found that phosphorylation of Tyr appears to have a n effect on the equilibrium ratio of cis-trans isomerization and decreases the cis content.