p21 Exploits Residue Tyr151 as a Tether for High-Affinity PCNA Binding.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Proliferating cell nuclear antigen (PCNA) is a key protein involved in DNA replication, repair, and cell cycle control.
• PCNA's overexpression in cancer cells makes it a significant pharmaceutical target.
• PCNA interacts with other proteins via a conserved motif known as the PCNA interacting protein (PIP)-box.

Purpose of the Study:

• To investigate the role of the hydroxyl group of tyrosine at position 151 (Tyr151) in the p21 PIP-box peptide.
• To understand how this specific residue affects the binding affinity between p21 and PCNA.

Main Methods:

• Cocrystallography was used to determine the structure of PCNA bound to a mutant p21 PIP-box peptide (p21Tyr151Phe).
• Isothermal titration calorimetry (ITC) was employed to quantify the binding thermodynamics.

Main Results:

• The p21Tyr151Phe mutant peptide exhibited a 3-fold lower binding affinity for PCNA compared to the wild-type.
• Significant differences in the entropy and enthalpy of binding were observed.
• Structural analysis revealed that the loss of the hydroxyl group disrupts hydrogen bonding and alters the interaction within the PCNA binding pocket.

Conclusions:

• The hydroxyl group of Tyr151 in p21 is essential for optimal binding to PCNA, acting as a critical tethering point.
• This interaction influences peptide interface packing and surface recognition, thereby modulating binding affinity.
• Understanding these molecular interactions can inform the design of novel therapeutics targeting PCNA.