Summary (modified parent R35 award) We investigate E3-ubiquitin ligase regulated protein turnover networks in various membrane compartments in mammalian cell systems. Thus, enrichment and purification of various membrane compartments from mammalian cells is essential for understanding the compartment specific protein turn over. Moreover, our understanding of mechanisms of membrane protein turnover networks and their phenotypes regulated by UPS remains sketchy. We engineer mammalian cell systems with various cell-genetic techniques to expose the cell fitness phenotypes, such as cell proliferation and survival, impacted by membrane anchored E3- ubiquitin ligases. Therefore, assays for the measurements of accurate cell number and cell size, in parent and engineered mammalian cells, are indispensable for the cell fitness studies.