Substrate specificity of CDK2-cyclin A. What is optimal?

Area of Science:

• Molecular Biology
• Enzymology
• Protein Kinase Signaling

Background:

• The optimal substrate sequence for cyclin-dependent kinases (CDKs) is known, but physiological substrates often deviate.
• Suboptimal phosphorylation sites in CDK substrates are frequently associated with a cyclin-binding (Cy) motif.
• The Cy motif is thought to compensate for reduced catalytic efficiency at suboptimal phosphorylation sites.

Purpose of the Study:

• To investigate the kinetic basis of substrate recognition by CDK2-cyclin A.
• To understand how the Cy motif influences substrate binding and catalysis with optimal and suboptimal phosphorylation sequences.

Main Methods:

• Kinetic analysis of CDK2-cyclin A with synthetic peptide substrates.
• Comparison of catalytic efficiency (kcat/Km), substrate binding (Km), and turnover (kcat) for different substrate motifs.
• Investigation of the interplay between phosphorylation site sequence and the Cy motif.

Main Results:

• In the optimal motif, Pro+1 enhances binding and catalysis, while Lys+2 and Lys+3 primarily enhance binding.
• The Cy motif enhances catalytic efficiency with suboptimal sequences (Lys+2 to Pro) by increasing affinity.
• When fused to optimal sequences, the Cy motif minimally enhances efficiency due to high affinity impeding catalysis.

Conclusions:

• Suboptimal phosphorylation site determinants in cellular substrates are kinetically compensated by the Cy motif.
• The Cy motif's effect depends on the underlying phosphorylation site sequence, modulating binding and turnover.
• These findings offer kinetic insights into the selection of specificity determinants for cellular substrate phosphorylation.