Substrate dependent regulation of Drosophila F-box protein

F-box proteins are specificity factors that target proteins for ubiquitin-mediated protein degradation. The F-box protein acts as a bridging protein, forming a specific protein interaction with its substrate, and linking it to the rest of the protein degradation machinery. Each F-box protein targets multiple substrates for degradation, and each organism has multiple F-box proteins that share a common degradation machine. Given this regulatory network for protein degradation, the question arises as to how the F-box proteins are regulated to ensure efficient protein degradation when substrates are presented requiring degradation. Are F-box proteins constitutively available? Or is their availability modulated according to the needs for substrate degradation? Because they share a common degradation machinery, is there a need to balance the expression of different F-box proteins so that no individual F-box unnecessarily ties up the shared machine? We have started to investigate these questions by examining the regulation of Drosophila Partner of paired (Ppa), an F-box protein known previously to target the Paired (Prd) transcription factor for degradation, and shown in this study to also target Even-skipped (Eve), another transcription factor. Protein expression studies revealed that the Ppa protein is up-regulated in response to elevated levels of its substrates. Elevated expression of Paired or Even-skipped substrates using transgenic constructs led to up-regulation of Ppa protein levels. Cycloheximide injection experiments suggest that the up-regulation of Ppa is controlled through protein stabilization. This provides an efficient mechanism for rapidly increasing the levels of an F-box protein in response to substrates requiring degradation. In the absence of substrates, there is high turnover of the F-box protein, leading to reduced expression in a context where its function is not needed.