Subunit counting in membrane-bound proteins

The subunit number and stoichiometry of membrane-bound proteins are difficult to determine without disrupting their membrane environment. Here we describe a single-molecule technique for counting subunits of proteins in live cell membranes by observing bleaching steps of GFP fused to a protein of interest. After testing the method with proteins of known stoichiometry expressed in Xenopus laevis oocytes, we resolved the composition of NMDA receptors composed of NR1 and NR3 subunits.