A flexible activin explains the membrane-dependent cooperative assembly of TGF-beta family receptors.
A new crystal structure of activin in complex with the extracellular domain of its type II receptor (ActRIIb-ECD) shows that the ligand exhibits an unexpected flexibility. The motion in the activin dimer disrupts its type I receptor interface, which may account for the disparity in its affinity for type I versus type II receptors. We have measured the affinities of activin and its antagonist inhibin for ActRIIb-ECD and found that the affinity of the 2-fold symmetric homodimer activin for ActRIIb-ECD depends on the availability of two spatially coupled ActRIIb-ECD molecules, whereas the affinity of the heterodimer inhibin does not. Our results indicate that activin's affinity for its two receptor types is greatly influenced by their membrane-restricted setting. We propose that activin affinity is modulated by the ligand flexibility and that cooperativity is achieved by binding to two ActRII chains that immobilize activin in a type I binding-competent orientation.
Greenwald
Jason
J
Structural Biology Laboratory, The Salk Institute, La Jolla, CA 92037, USA.
Vega
Mark E
ME
Allendorph
George P
GP
Fischer
Wolfgang H
WH
Vale
Wylie
W
Choe
Senyon
S
D028341:Activins / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism*; D008024:Ligands; D051379:Mice; D017434:Protein Structure, Tertiary; D018125:Receptors, Transforming Growth Factor beta / Q000096:biosynthesis*; D016212:Transforming Growth Factor beta / Q000378:metabolism
D008024:Ligands; D018125:Receptors, Transforming Growth Factor beta; D016212:Transforming Growth Factor beta; D028341:Activins