Conformation of Human Microtubule Associated Protein-Tau

Abstract

Tau mainly functions to bind and stabilize microtubules. Tau is classified as a natively unfolded protein and lacks of secondary structural elements. Tau aggregates to insoluble fibers called paired helical filaments in Alzheimer disease. The reaction of certain antibodies with discontinuous epitopes has suggested the possibility that tau shows some global conformations. Also, molecular forces that contribute to the stability of PHFs are not well understood. This present study is aimed to a better understanding of the structure of soluble tau and PHFS. The results are summarized as follow: (i) Soluble tau remained as random coil at various pHs despite charge minimization and at high ionic strength as judged by CD. In contrast, elevated temperature induces beta-structure and alcohol tends to slightly alter the conformation of tau. Overall, these results highlight the low hydrophobic content of tau being the main cause for its unfolded nature. (ii) The global folding of tau was examined by fluorescence resonance energy transfer (FRET). High FRET within the second and third repeat of the microtubule binding domain indicated a local compaction. The results also showed that the repeat domain of tau comes into close vicinity of the C-terminal tail, but not to the N-terminus. Surprisingly the N- and the C-terminal ends showed FRET. Based on this, tau contains preferred long range interactions between the repeat domain and the C-terminus and between the N- and C-terminus ("paperclip"). The interactions between the domains are obliterated by denaturation in GdnHCl, pointing to fluctuating chain that otherwise is almost as flexible as a denatured protein. This folding agrees well with the reactivities of certain antibodies (Alz50, MC1, Tau66 and SMI34) that detect abnormal tau in early stages of Alzheimers disease, suggesting that a stabilization of the folded state could have pathological consequences. (iii) Tau aggregation in the presence of heparin is more efficient at pH just below its pI that minimizes net charge of tau. Temperature dependence and salt dependence of tau aggregation suggest a strong influence of ionic interactions. Nevertheless, hydrophobic patches appear in tau aggregates as measured by ANS fluorescence. iv) The denaturation of PHFs at mild concentrations of GdnHCl as well as the partial dissolution of PHFs at extreme pHs points to ionic nature of interactions within PHFs. In contrast, PHFs are stable at elevated temperature, at high salt concentrations and in alcohols like isopropanol showing that once PHFs are formed the interactions become stabilized. These observations could form a basis for elucidating the molecular architecture of PHFs.