Fig. 7

Model: Heterogeneity of Wnt complex formation and diffusion range. Wnt trimer is the smallest unit of the HMW complexes. Both the trimer and trimer-assembled larger complexes appear to exist in the extracellular milieu, although it is uncertain as to when the timing of the trimer formation, as well as that of the assembly to the larger HMW complexes, occurs during the process of Wnt secretion. The large HMW complex is less mobile, probably interacting with the plasma membrane, resulting in restriction of the Wnt diffusion range. Some Wnt molecules in the HMW form can be dissociated by local interaction with Frizzled receptor (Fzd), resulting in short-range signal (local action). In contrast, the larger HMW complexes, probably as well as the trimer itself, can also be dissociated by interaction with soluble Wnt-binding protein, such as sFRP2. By this dissociation, Wnt becomes more mobile; and its diffusion range is thus expanded (diffusible action)