A minimum folding unit in the ankyrin repeat protein p16(INK4)

Area of Science:

• Protein structure and folding
• Molecular biology
• Biochemistry

Background:

• Ankyrin repeats are common protein motifs crucial for protein structure.
• The cyclin-dependent kinase inhibitor p16 (p16) contains ankyrin repeats and functions as a tumor suppressor.
• Understanding the minimal folding unit of ankyrin repeat proteins is essential for protein engineering.

Purpose of the Study:

• To identify the smallest autonomous folding unit within the ankyrin repeat protein p16.
• To characterize the structural and thermodynamic properties of this minimal folding unit.

Main Methods:

• Proteolysis studies and computational predictions to identify potential folding fragments.
• Far-UV circular dichroism to assess secondary structure.
• Thermal and urea-induced denaturation to determine thermodynamic stability.
• Proton-nitrogen 2D Nuclear Magnetic Resonance (NMR) to study protein structure.

Main Results:

• A fragment of p16, termed p16C, comprising the third and fourth ankyrin repeats, was identified as an independently folding unit.
• p16C exhibits significant alpha-helical secondary structure and cooperative, reversible thermal denaturation.
• The free energy of unfolding for p16C was estimated at 1.7 kcal/mol.
• Single ankyrin repeat peptides derived from p16C were unstructured, indicating p16C is the minimal folding unit.

Conclusions:

• The p16C fragment represents the smallest known independently folding ankyrin repeat module.
• The two-ankyrin repeat fold is proposed as the minimum structural unit for ankyrin repeat proteins.
• This research has implications for understanding protein folding principles and for protein engineering applications.