Detection of Isoforms Differing by a Single Charge Unit in Individual Cells.

To measure protein isoforms in individual mammalian cells, we report single-cell resolution isoelectric focusing (scIEF) and high-selectivity immunoprobing. Microfluidic design and photoactivatable materials establish the tunable pH gradients required by IEF and precisely control the transport and handling of each 17-pL cell lysate during analysis. The scIEF assay resolves protein isoforms with resolution down to a single-charge unit, including both endogenous cytoplasmic and nuclear proteins from individual mammalian cells.