Development of temperature-sensitive mutants of the Drosophila melanogaster P-TEFb (Cyclin T/CDK9) heterodimer using yeast two-hybrid screening.

P-TEFb complex, a heterodimer of the kinase CDK9 and Cyclin T, is a critical factor that stimulates the process of transcription elongation. Here, we explored a fast and large-scale screening method to induce a temperature-dependent conditional disruption of the CDK9/Cyclin T interaction and developed an assay to validate their mutant phenotypes in a biological context. First, we used the yeast two-hybrid system to screen Drosophila melanogaster Cyclin T mutants at a large scale for temperature or cold sensitive (TS or CS) CDK9 interaction phenotypes. The isolated P-TEFb TS mutants were then expressed in Drosophila cells and were investigated for their effects on Drosophila hsp70 transcriptional activity. Our results showed that these P-TEFb TS mutants had a reduced level of hsp70 transcription at restrictive temperatures. A model structure of the Cyclin T and CDK9 complex suggested that the key TS mutations were found within the α2- and α3-helices at the interface of the complex, which may disrupt the binding of Cyclin T to CDK9 directly or indirectly by affecting the conformation of Cyclin T. The yeast two-hybrid-based screening strategy described here for isolating TS or CS interaction phenotypes can be directly applicable to other complexes in higher organisms. The use of TS or CS mutants will enable a 'real-time and reversible perturbation' restricted to specific protein-protein interactions, providing a mechanistic insight into the biological process mediated by a target complex.