a, Intracellular protein concentrations are estimated against reference solutions containing known concentrations of purified YFP. We transferred solutions of purified YFP into the same plate where we inoculated cells for imaging. This enabled us to relate the fluorescence emitted from YFP solutions to that emitted from cells. b, The signal measured by the confocal spinning disk microscope increases linearly with YFP concentration. The equation inferred from linear regression enabled us to convert fluorescence arbitrary units into YFP molarity (1 nM = 0.998 fluorescence arbitrary units − 0.16). c, We used the equation so obtained to transform the median intracellular fluorescence signal into homo-oligomer concentrations. Bars show the population median with associated standard error. We initially used the GPD promoter, which gave concentrations in the sub-micromolar range, and subsequently also used a CYC promoter to express four randomly chosen proteins. Expression with the CYC promoter gave concentrations in the range 10–50 nM, at which mutants also underwent supramolecular assembly (images are shown in Supplementary Table 2).