FIGURE 9.

Models for clustering mechanisms. (A–B) A simple like-associates-with-like model would suggest that increasing amounts of labeled ligand-receptor complexes (D + A) results in increasing local cluster density. (C–D) A saturation-cluster model explains the negative dependence of local cluster density on increasing D + A. Left to right shows a doubling of the total number of donor and acceptor molecules (D + A), i.e., n to 2n. The value of s indicates an estimation of the local cluster density as described in Fig. 8. In D, unorganized donor and acceptor molecules, outside the saturated cluster, have the effect of lowering the overall density. Each square represents a pixel area, and asterisks indicate the distribution of membrane-bound ligand-receptor complexes.