A gradient of BMP activity specifies dorsal-ventral fates in early Xenopus embryos

BMP-4 is an extracellular signalling molecule belonging to the TGF-P superfamily that plays a central role in dorsoventral patterning in vertebrate gastrulae. We review the evidence indicating that BMP-4 acts as a morphogen, specifying dorsoventral positional values in concentration-dependent manner. An activity gradient of BMP-4 is established not by Simple diffusion from a localised source, but by diffusion of inhibitory binding proteins that act on a uniform level of BMP-4 protein. These in turn are regulated by the activity of tolloid-related metalloproteases, such as Xenopus xolloid and zebrafish tolloid.