In contrast, the RGG domain of Sbp1, Sbp1RGG, interacts with Pab1 directly in the absence of 5′UTRPab1 (Fig. 3B, lane 2). Unsurprisingly, neither the presence of an equal stoichiometric amount of 5′UTRPab1 nor an addition of RNase A in the presence of 5′UTRPab1 alters the protein–protein interaction (Fig. 3B, cf. lanes 2, 3, and 4). A replacement of the RGG domain in Sbp1 with 17 glycine–serine repeats (GS repeats) abolishes the interaction between Sbp1 and Pab1 regardless of the presence of 5′UTRPab1 (Fig. 3C, lanes 2, 3, and 4). Furthermore, Pab1 interacts with the Sbp1–5′UTRPab1 complex (Fig. 3D, lane 3), whereas Sbp1 does not interact with the Pab1–5′UTRPab1 complex (Fig. 3D, lane 4). These observations demonstrate that the RNA binding of Sbp1, not Pab1, is important for interactions between Pab1 and Sbp1. As such, our results indicate that exposing the RGG domain of Sbp1 by the RNA binding is required for Sbp1 to bind Pab1.