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.