Conclusion
Through this study, we tried to explore the effects of N-glycosylation on the structural dynamics of tau protein by glycosylating one of the PTM site, i.e., Asn-359 using a microsecond long all atom molecular dynamics simulation. Analysis of system stability showed that each tau system were quickly equilibrated, and remained stable throughout the trajectory without any major instabilities with the two extreme end regions of the protein being the most flexible compared to the middle regions. Protein compactness analysis and essential dynamics further showed that when the peptide was N-glycosylated it remained most compact and had a folded state, while in the unmodified state, the peptide was less compact and had an extended conformation. Further insights were obtained from the secondary structure analysis, which showed that the glycosylated tau, had a higher beta sheet propensity as compared to the unmodified peptide. Free energy landscape analysis revealed the folding process of the unmodified tau is rugged and unstable, while that of N-glycosylated and phosphorylated systems were comparatively stable and structural Analysis of the local energy minima structures revealed that glycosylated energy minimum structures had a higher amount of beta sheets while the unmodified tau had zero beta sheets, further justifying that N-glycosylation induces the tau protein to fold. Residue interaction network analysis revealed the locations in the protein which helps in the folding process. BC analysis revealed Phe378 & Lys353, as the functionally important residues for the folding process, while the Phe378, Lys370 & Lys343 helped stabilize the folded state of the protein.
Tau hyperphosphorylation is already an established factor which induces tau protein folding and aggregation. Comparative analysis of our results of the glycosylated system (Tau_glyc) and phosphorylated system (Tau_phos) suggest that both showed similar behavior, both existing in a folded conformation having comparatively high beta sheets regions. But Tau_p356 & Tau_p352 doesn’t induce much folding. Our analysis and results are in line with the in-vitro studies conducted by Losev & coworkers as our study confirmed that N-glycosylation at Asn-359 is inducing tau protein folding. Also, the high beta sheet content found in the glycosylated tau shows it is having high aggregation propensity as compared to the unmodified non glycosylated tau. We believe that the results of this analysis can be further be used for the development of novel tau aggregation inhibitors which can target the functionally important residues and their residue specific interactions. Moreover, the process of N-glycosylation is initiated by oligosaccharyltransferase enzyme and the pathogenic role played by this enzyme in AD pathology and its druggability together with the exact mechanisms by which tau is N-glycosylated can be further explored. We also believe that, this study can help to explore the development of novel therapeutic molecules targeted at blocking the N-glycosylation induced tau misfolding and aggregation.