FIG. 3.

Myostatin binds to ActRIIB and signals through a heteromeric receptor complex composed of either ALK4 or ALK5. (A to D) COS-1 cells were transiently transfected with various type II receptors (A) or individual type I receptors (B) or the type II receptor ActRIIB together with the type I receptors ALK1 to ALK6 (A1 to A6) (C and D) as indicated. The cells were incubated with [¹²⁵I]myostatin (0.03 nM [A and C] or 1 nM [B]) in the presence or absence of unlabeled myostatin (15 nM). Total cell lysates were separated by SDS-PAGE, and ¹²⁵I was detected by autoradiography (A, C, and D) or phosphorimaging (B). Expression of receptor protein in total cell lysates was determined by immunoblotting with anti-HA antibody (B). (E) Mv1Lu cell derivatives, RIB L17 or DR 27 cells, were transiently transfected with the 3TP-Lux reporter either with or without the type I receptors ALK4 or ALK5 (upper panel) or the type II receptor TβRII (lower panel). The fold induction of luciferase activity in cells treated with myostatin, TGF-β, or activin relative to controls was determined.