| The development of a multi-cellular organism is driven by an increase in cellular growth (increase in cell size) and cell proliferation (increase in cell number through division). In order for these processes to coordinate smoothly, appropriate genes must be expressed and terminated. The termination of gene expression is often regulated through protein degradation mediated by the Ubiquitin Proteasome System. In this system, target proteins are marked by polyubiquitination and are shuttled to be degraded by a protease complex, the proteasome. Substrate ubiquitination is carried out by E3 ubiquitin ligases that each recognizes limited number of targets. My thesis work focused on a member of a large ubiquitin ligase family, Cullin Ring Ligase 4, and its role during cell growth and proliferation. Mammals have two closely related Cul4s, Cul4A and Cul4B, complicating the study of CRL4 function. However, Drosophila melanogaster has one Cul4, making this model organism more accessible.During my thesis, I used previously generated alleles of Cul4 and Ddb1 and demonstrated CRL4 mediated destruction of an important inhibitor of cellular growth, TSC2/Gigas. I also studied CRL4 function in replication control using a mutant substrate, Cdt1/Dup, that cannot be recognized by CRL4. We found that CRL4-mediated destruction of Cdt1/Dup is crucial for mitotic cell cycle progression in embryos. However, CRL4 regulation is dispensable in endocycles during oogenesis. Taken together, these results suggest tissue specific contribution of CRL4 in replication control. |
