Split reporter protein-based genetic section systems are widely used to identify and characterize protein-protein interactions (PPI). The assembly of split markers that antagonize toxins, rather than required for synthesis of missing metabolites, facilitates the seeding of high density of cells and selective growth. Here we present a newly developed split chloramphenicol acetyltransferase (split-CAT) -based genetic selection system. The N terminus fragment of CAT is fused downstream of the protein of interest and the C terminus fragment is tethered upstream to its postulated partner. We demonstrate the system's advantages for the study of PPIs. Moreover, we show that co-expression of a functional ubiquitylation cascade where the target and ubiquitin are tethered to the split-CAT fragments results in ubiquitylation-dependent selective growth. Since proteins do not have to be purified from the bacteria and due to the high sensitivity of the split-CAT reporter, detection of challenging protein cascades and post-translation modifications is enabled. In addition, we demonstrate that the split-CAT system responds to small molecule inhibitors and molecular glues (GLUTACs). The absence of ubiquitylation-dependent degradation and deubiquitylation in E. coli significantly simplify the interpretation of the results. We harnessed the developed system to demonstrate that like NEDD4, UBE3B also undergoes self-ubiquitylation-dependent inactivation. We show that self-ubiquitylation of UBE3B on K665 induces oligomerization and inactivation in yeast and mammalian cells respectively. Finally, we showcase the advantages of split-CAT in the study of human diseases by demonstrating that mutations in UBE3B that cause Kaufman oculocerebrofacial syndrome exhibit clear E. coli growth phenotypes.

Cell Biology Program The Hospital for Sick Children and Biochemistry Department University of Toronto Toronto ON Canada

Institute of Cell Biology and Neurobiology Charité Universitätsmedizin Berlin Corporate Member of Freie Universität Berlin and Humboldt Universität zu Berlin Charitéplatz 1 10117 Berlin Germany

Institute of Medical Genetics Tomsk National Research Medical Center Neuroscience Lobachevsky University of the Russian Academy of Sciences Nizhny Novgorod pr Gagarina 24 Nizhny Novgorod Russia

Novartis Institutes for Biomedical Research 250 Massachusetts Ave Cambridge MA 02139 USA

Sagol School of Neuroscience Tel Aviv University Tel Aviv Israel

School of Neurobiology Biochemistry and Biophysics The George S Wise Faculty of Life Sciences Tel Aviv University Ramat Aviv 69978 Tel Aviv Israel

The Shmunis School of Biomedicine and Cancer Research Tel Aviv University Ramat Aviv 69978 Tel Aviv Israel