Figure 8.

Model summarizing differences between synthetic MBNL proteins. MBNL-AA is a more active alternative splicing regulator while MBNL-BB is significantly weaker compared to MBNL-AB. These differences in activity are represented by the size of the arrows showing how each MBNL protein promotes exon inclusion / exclusion. In the context of the canonical arrangement of ZF domains within MBNL proteins (MBNL-AB), ZF1–2 binds YGCY motifs with high specificity while ZF3–4 has a modest preference for YGCY motifs but will sample and bind many motifs with similar affinities. This activity is represented by the dotted lines illustrating ZF3–4 binding to both canonical and non-canonical RNA motifs. MBNL-AA possesses two high-specificity ZF1–2 motifs driving RNA recognition and subsequently increased splicing regulation at lower protein concentrations. MBNL-BB has significantly reduced RNA binding specificity and samples many specific and non-specific RNA motifs. Due to the reduced motif recognition, regulatory sites are not bound until high concentrations of protein are present leading to an overall reduction in splicing regulation. Structures of domains shown here are derived from PDB ID 3D2N (ZF1–2) and PDB 3D2Q (ZF3–4) (33).