FIG 4.

Single particle tracking (SPT) detects short-lived erbB2-erbB3 heterodimers and longer-lived erbB3 homodimers. (A and B) Receptor dimerization behavior based on tracking of a pair of QD-tagged receptors over time. Shown are representative plots of separation distances for an erbB2-erbB3 heterodimer on CHO cells expressing exogenous HA-erbB2 and untagged erbB3 (A) and endogenous erbB3 receptors interacting on SKBR3 cells (B). Horizontal lines indicate state calls (right axis, dimer, coconfined, or separated receptors) as determined by a Viterbi algorithm. Insets show images of pairs at specific time frames. Scale bar, 1 μm. (C) Diagram illustrating interaction states of erbB3 tracked with two colors of QD-HRG and with different levels of unlabeled erbB2 (gray spheres). (D to I) Histograms plotting the distribution of dimer lifetimes for receptor pairs as indicated (diagrams). k_off values are predicted from the HMM analysis.