Evidence for polyproline II helical structure in short polyglutamine tracts

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Expanded polyglutamine sequences in proteins are linked to nine neurodegenerative diseases, including Huntington's disease.
• Protein aggregation, specifically beta-sheet-rich deposits, is a hallmark of these diseases.
• The initial structural state of monomeric polyglutamine, a precursor to aggregation, remains poorly understood.

Purpose of the Study:

• To investigate the structural propensity of glutamine residues in monomeric polyglutamine.
• To determine if glutamine residues favor specific conformations, such as the polyproline II (P(II)) helix.
• To elucidate the starting state of polyglutamine relevant to protein aggregation.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was employed to study polyglutamine structures.
• Circular dichroism (CD) spectroscopy was used to analyze secondary structure content.
• Computational studies provided a basis for experimental investigation.

Main Results:

• NMR and CD spectroscopy confirmed a significant propensity for glutamine residues to adopt the P(II) helical conformation.
• This P(II) propensity was observed in short glutamine tracts (up to 15 residues).
• While other structures are possible, P(II) is a favored conformation for monomeric polyglutamine.

Conclusions:

• Monomeric polyglutamine exhibits a notable preference for the P(II) helical structure.
• Understanding these initial monomeric structures is vital for comprehending the polyglutamine aggregation pathway.
• The P(II) conformation is not necessarily a direct precursor to aggregation but represents a key structural feature.