A Flow Cytometric Assay for the Study of E3 Ubiquitin Ligase Activityb

Abstract

BACKGROUND: Current methods for monitoring E3 ubiquitin ligase activity in cell culture or in vivo are limited. As a result, the degradation of cellular targets by many E3 ubiquitin ligases in live cells has not yet been examined. METHODS: A target of an E3 ubiquitin ligase was expressed as a fluorescently labeled protein in cell culture. If the E3 ubiquitin ligase mediates the degradation of a target protein in cell culture, it is expected that the target will show a reduced fluorescence signal by FCM analysis. We initially used the E3 ubiquitin ligase, herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) and one of its targets, promyelocytic leukemia (PML) protein, to determine the feasibility of our approach. Cells expressing a PML-GFP fusion protein were selected by cell sorting and infected with an adenoviral vector expressing ICP0. RESULTS: In contrast to mock-infected cells, only PML-GFP-expressing cells infected with the ICP0 adenoviral vector led to a significant decrease in the fluorescence signal of PML-GFP when examined by fluorescence microscopy and FCM analysis. CONCLUSIONS: Using HSV-1 ICP0 as a paradigm, it is possible to examine the live activity of an E3 ubiquitin ligase (via one of its targets) in cell culture with FCM analysis.