FIGURE 1 

Determining sizes of receptor signaling teams from signaling dose–response curves. Signaling-team size N, i.e. the number of receptor dimers per signaling team, may differ depending on the receptor modification state (glutamate (E) or glutamine (Q) at four specific receptor modification sites). Variable signaling-team sizes are illustrated schematically by membrane patches of trimers of dimers (blue circles) with signaling teams of two trimers (N=6 dimers) for Tar{QEEE}, or signaling teams of six trimers (N=18 dimers) for Tar{QQQQ}. When directly fitting the free-energy model of receptor activity to the noisy data, the fitted curves and model parameters vary widely, due to the large error bars (middle panel), preventing quantitative analysis of signaling-team sizes. However, use of principal component analysis (PCA) separates the reproducible variation of the data from the sampling noise, allowing quantitative evaluation of parameters, and revealing a systematic dependence of signaling-team size on receptor modification level in living cells.