Activin R2B

Activin isoforms and other members of the TGFβ superfamily exert their biological effects by binding to heteromeric complexes of a type I and a type II serinethreonine kinase receptor, both of which are essential for signal transduction. Seven type I and five type II receptors, including the two type I and the two type II activin receptors, designated ActRI (A), ActRIB, ActRII (A) and ActRIIB or activin R2B, have been cloned from mammals. Through alternative mRNA splicing, multiple ActRIIB isoforms can also be generated, adding to the complexity of the activin receptor system. Different activin isoforms bind with different highaffinities to the various type II isoforms. Type I activin receptors do not bind directly to activin, but will associate with the type II receptor activin complex and initiate signal transduction. Besides the activin isoforms, ActRII will also bind inhibin, BMP2 and BMP7 with lower affinities. ActRI can also bind and form signaling complexes with the BMP2/ 7bound BMPRII. Activin type II receptors are highly conserved. Human, mouse and rat type II activin receptors share greater than 98% amino acid sequence homology. Recombinant soluble activin type II receptors bind activin with high affinity, and are potent activin antagonists.