Single-Cell Native Mass Spectrometry of Human Erythrocytes

Native mass spectrometry (MS) enables the determination of the molecular mass of protein complexes. Generally, samples for native MS are isolated, purified, and prepared in volatile solutions. However, to understand the function of proteins in living cells, it is essential to characterize the protein complex as is, without isolation/purification of the protein, using the smallest possible amount of the sample. In the present study, we modified the "live single-cell MS" method, which has mainly been used in metabolomics, and applied it to observe hemoglobin directly sampled from human erythrocytes. By optimizing the experimental methods and conditions, we obtained native mass spectra of hemoglobin using only a single erythrocyte, which was directly sampled into a nanoelectrospray ionization emitter using a micromanipulator and microinjector system. That is, our method enables the analysis of ∼0.45 fmol of hemoglobin directly sampled from an erythrocyte. To our knowledge, this is the first report of native MS for endogenous proteins using a single intact human cell.