We have observed that the extracellular domain of T beta RI and protectin (CD59), an inhibitor of the membrane attack complex of complement, share structural features, a distinct spacing of ten cysteines and a C-terminal 'Cys-box'. Based on these common features and the recently determined NMR-structure of protectin, a three-dimensional model for the extracellular domain of T beta RI was constructed. After energy minimization and molecular dynamics simulation, a structure with four extending fingers (pes quattvordigitorum) and two clusters of charged residues was obtained. This model provides a view to the understanding of interactions between T beta RI, T beta RII and TGF beta during ligand recognition and signal transduction.