Dichotomous but stringent substrate selection by the dual-function Cdk7 complex revealed by chemical genetics

Area of Science:

• Molecular Biology
• Biochemistry
• Cell Biology

Background:

• Cyclin-dependent kinase 7 (Cdk7) is a crucial enzyme with a dual role.
• It acts as a CDK-activating kinase (CAK) for cell-cycle progression.
• It also functions as the RNA polymerase II (Pol II) CTD kinase within transcription factor IIH.

Purpose of the Study:

• To investigate the substrate specificity of Cdk7.
• To understand how Cdk7 distinguishes between its roles in cell-cycle control and transcription.
• To identify the molecular mechanisms underlying Cdk7's dual functions.

Main Methods:

• Creation of an analog-sensitive (AS) Cdk7 mutant capable of utilizing bulky ATP derivatives.
• Phosphorylation analysis of endogenous polypeptides in nuclear extracts using Cdk7-AS-cyclin H-Mat1.
• Identification and characterization of Cdk7 substrates.

Main Results:

• Cdk7-AS-cyclin H-Mat1 phosphorylated approximately 10-15 endogenous polypeptides.
• Seven substrates were identified, categorized into two distinct classes.
• Class 1 substrates (e.g., Pol II, Spt5) require fully activated Cdk7 and specific surrounding sequences.
• Class 2 substrates (e.g., other CDKs) are targeted by all active Cdk7 forms, dependent on remote substrate motifs.

Conclusions:

• Cdk7 achieves its distinct functions in cell-cycle regulation and transcription through specific, stringent substrate recognition modes.
• The enzyme's substrate specificity prevents promiscuity, ensuring precise control over cellular processes.
• This study elucidates the molecular basis for Cdk7's essential dual roles in eukaryotic cells.