Identification and Relative Quantitation of an Orphan G-Protein Coupled Receptor SREB2 (GPR85) Protein in Tissue Using a Linear Ion Trap Mass Spectrometer

SREB2 (GPR85) is an orphan G-protein coupled receptor (GPCR) whose function is unknown. We previously prepared a SREB2-overexpressing transgenic mouse for functional analysis but were unable to confirm SREB2 protein expression level by immunochemical or biochemical methods. In this article, we report mass spectrometric identification and relative quantitative analysis of SREB2 in the forebrains of transgenic and wild type mice using nanoliquid chromatography coupled with a linear ion-trap mass spectrometer. By analyzing Chinese hamster ovary (CHO) cells overexpressing the SREB2 gene, we identified a proteotypic SREB2 peptide, GPTPPTLLGIR. Using a stable isotope-labeled analog as an authentic peptide for protein identification and as an internal control for relative quantitation, SREB2 was directly identified from the membrane fraction of forebrains from wild type and SREB2 transgenic mice. SREB2 protein expression level in the transgenic mouse was estimated to be 3-fold higher than that in the wild type littermate.