Molecular basis for phosphorylation-dependent, PEST-mediated protein turnover

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Proteasomal degradation is crucial for protein homeostasis.
• Phosphorylation of PEST sequences regulates protein turnover.
• The papillomavirus E2 protein is essential for viral gene expression and replication.

Purpose of the Study:

• To investigate the structural basis of PEST sequence-mediated degradation of the papillomavirus E2 protein.
• To determine the role of phosphorylation in modulating E2 protein stability and turnover.
• To elucidate the mechanism by which the proteasome machinery recognizes and degrades the E2 protein.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to analyze the structure of a peptide fragment from the E2 PEST region.
• The effects of pH and phosphorylation on peptide structure were investigated.
• Point mutations were introduced into the peptide and E2 protein to assess their impact on stability and half-life in vivo.
• Thermodynamic stability of peptides was correlated with E2 protein half-life.

Main Results:

• The E2 PEST region peptide exhibits pH-dependent structural transitions between polyproline II and alpha helix conformations.
• Phosphorylation, particularly at serine 301, disrupts the peptide's structure.
• Mutations altering peptide stability showed a strong correlation with in vivo E2 protein half-life.
• The PEST region possesses marginal stability finely tuned by phosphorylation.

Conclusions:

• Conformational stability of the PEST sequence, modulated by phosphorylation, is the primary determinant of E2 protein degradation by the proteasome.
• Degradation is controlled by structural changes, not solely by the recognition of a phosphate modification.
• This mechanism highlights a sophisticated regulatory strategy for controlling viral protein turnover.