Protein-protein interactions are often mediated by small domains that recognize solvent-exposed peptide sequences. Deciphering the recognition code for these adapter domains is an important step in the understanding of multi-protein assemblies. Here, we investigate the sequence requirements for the CD2BP2-GYF domain, a proline-rich sequence binding module previously shown to be involved in T cell signaling. We show that the signature (R/K/G)XXPPGX(R/K) defines a preferred peptide-binding motif that is present in several proteins of the splicing machinery. Specifically, the core small nuclear ribonucleoprotein, SmB/B', contains several PPP-PGMR motifs that interact with the CD2BP2-GYF domain in vitro and in vivo. The colocalization of CD2BP2 and SmB proteins in the nucleus of Jurkat T cells and HeLa cells suggests a function of the GYF domain of CD2BP2 in mediating protein-protein interactions within the spliceosome.