Model of receptor activation. The low-affinity binary complex consists of GM-CSF (magenta) bound to GMRα (red). Interaction with free βc (blue and green) forms the high-affinity hexamer complex. Dodecamer (or higher order) complexes form by lateral aggregation of hexamer complexes to form a fully competent signaling complex. Jak2 associated with βc (data not shown) is able to dimerize and transphosphorylate in the dodecamer complex, but not in the hexamer complex.