Binding Properties of the Transforming Growth Factor-ß Coreceptor Betaglycan: Proposed Mechanism for Potentiation of Receptor Complex Assembly and Signaling.

ABSTRACT

Transforming growth factor (TGF) ß1, ß2, and ß3 (TGF-ß1-TGF-ß3, respectively) are small secreted signaling proteins that each signal through the TGF-ß type I and type II receptors (TßRI and TßRII, respectively). However, TGF-ß2, which is well-known to bind TßRII several hundred-fold more weakly than TGF-ß1 and TGF-ß3, has an additional requirement for betaglycan, a membrane-anchored nonsignaling receptor. Betaglycan has two domains that bind TGF-ß2 at independent sites, but how it binds TGF-ß2 to potentiate TßRII binding and how the complex with TGF-ß, TßRII, and betaglycan undergoes the transition to the signaling complex with TGF-ß, TßRII, and TßRI are not understood. To investigate the mechanism, the binding of the TGF-ßs to the betaglycan extracellular domain, as well as its two independent binding domains, either directly or in combination with the TßRI and TßRII ectodomains, was studied using surface plasmon resonance, isothermal titration calorimetry, and size-exclusion chromatography. These studies show that betaglycan binds TGF-ß homodimers with a 11 stoichiometry in a manner that allows one molecule of TßRII to bind. These studies further show that betaglycan modestly potentiates the binding of TßRII and must be displaced to allow TßRI to bind. These findings suggest that betaglycan functions to bind and concentrate TGF-ß2 on the cell surface and thus promote the binding of TßRII by both membrane-localization effects and allostery. These studies further suggest that the transition to the signaling complex is mediated by the recruitment of TßRI, which simultaneously displaces betaglycan and stabilizes the bound TßRII by direct receptor-receptor contact.