Quantitative Analysis of Human Red Blood Cell Proteome.

Red blood cells (RBCs) are the most abundant cell type in the human body. RBCs and, in particular, their plasma membrane composition have been extensively studied for many years. During the past decade proteomics studies have extended our knowledge on RBC composition; however, these studies did not provide quantitative insights. Here we report a large-scale proteomics investigation of RBCs and their "white ghost" membrane fraction. Samples were processed using the multienzyme digestion filter-aided sample preparation (MED-FASP) and analyzed using Q-Exactive mass spectrometer. Protein abundances were computed using the total protein approach (TPA). The validation of the data with stable isotope-labeled peptide-based protein quantification followed. Our in-depth analysis resulted in the identification of 2650 proteins, of which 1890 occurred at more than 100 copies per cell. We quantified 41 membrane transporter proteins spanning an abundance range of five orders of magnitude. Some of these, including the drug transporter ABCA7 and choline transporters SLC44A1 and SLC44A2, have not previously been identified in RBC membranes. Comparison of protein copy numbers assessed by proteomics showed a good correlation with literature data; however, abundances of several proteins were not consistent with the classical references. Because we validated our findings by a targeted analysis using labeled standards, our data suggest that some older reference data from a variety of biochemical approaches are inaccurate. Our study provides the first "in-depth" quantitative analysis of the RBC proteome and will promote future studies of erythrocyte structure, functions, and disease.