Stable Isotope Labeling by Amino Acids in Cell Culture and Differential Plasma Membrane Proteome Quantitation Identify New Substrates for the MARCH9 Transmembrane E3 Ligase

The regulation of cell surface receptor expression is essential for immune cell differentiation and function. At the plasma membrane ubiquitination is an important post-translational mechanism for regulating expression of a wide range of surface proteins. MARCH9, a member of the RING-CH family of transmembrane E3 ubiquitin ligases, down-regulates CD4, major histocompatibility complex-I (MHC), and ICAM-1 in lymphoid cells. To identify novel MARCH9 substrates, we used high throughput flow cytometry and quantitative mass spectrometry by stable isotope labeling by amino acids in cell culture (SILAC) to determine the differential expression of plasma membrane proteins in a MARCH9-expressing B cell line. This combined approach identified 13 potential new MARCH9 targets. All of the SILAC-identified targets for which antibodies were available were subsequently confirmed by flow cytometry, validating the proteomics results. A close correlation (r(2) = 0.93) between -fold down-regulation as determined by SILAC and flow cytometry was found, with no false positive hits detected. The potential new MARCH9 substrates cover a wide range of functions and include receptor-type protein-tyrosine phosphatases (e.g. PTPRJ/CD148) as well as Fc γ receptor IIB (CD32B), HLA-DQ, signaling lymphocytic activation molecule (CD150), and polio virus receptor (CD155). The identification of plasma membrane targets by SILAC with confirmation by flow cytometry represents a novel and powerful approach to analyze changes in the plasma membrane proteome.