STK40 Is a Pseudokinase that Binds the E3 Ubiquitin Ligase COP1

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Serine/threonine kinase 40 (STK40) is implicated in various physiological processes.
• Its precise biological activity and mechanism of action remain largely uncharacterized.
• STK40 is recognized as a distant homolog of Tribbles-family proteins.

Purpose of the Study:

• To investigate the interaction between STK40 and Constitutive Photomorphogenic Protein 1 (COP1).
• To elucidate the structure-function relationships of STK40, particularly its kinase domain.
• To determine if STK40 functions as a pseudokinase and explore its role as a COP1 adaptor.

Main Methods:

• Co-immunoprecipitation assays to confirm STK40-COP1 interaction.
• Crystal structure determination of the STK40 kinase homology domain at 2.5 Å resolution.
• ATP-binding assays to assess kinase activity.

Main Results:

• STK40 directly interacts with COP1 via a C-terminal sequence.
• The crystal structure reveals STK40 is a pseudokinase due to substitutions in conserved residues, preventing ATP binding.
• Despite structural divergence from Trib1, STK40 exhibits functional similarities as a COP1 adaptor.

Conclusions:

• STK40 functions as a pseudokinase and acts as a COP1 adaptor.
• The findings highlight functional parallels between STK40 and Tribbles-family proteins.
• This study provides structural insights into STK40's mechanism of action as a COP1-binding protein.