SARA permits the binding of the R-SMAD to the L45 region of the Type I receptor. SARA orients the R-SMAD such that serine residue on its C-terminus faces the catalytic region of the Type I receptor.
The SARA protein enables the binding of R-SMAD to the L45 region of the Type I receptor.  This interaction is facilitated by SARA, which positions the R-SMAD so that a serine residue on its C-terminus interacts with the catalytic domain of the Type I receptor.