Swapped domain orders in ZO-1 PDZ3 fusion proteins - implications for binding of established and novel targets

PDZ domains play key roles in mediating protein-protein interactions by recognizing short PDZ-binding motifs, typically at the C-termini of target proteins. Zonula occludens 1 (ZO-1) is a scaffolding protein that links tight junction proteins to the actin cytoskeleton, and contains three PDZ domains. Here, we focus on its third PDZ (PDZ3_ZO-1) domain, which interacts with the C-terminus of junctional adhesion protein A as well as connexin 45. To investigate how the domain context of the PDZ3_ZO-1 domain affects its folding and function, we previously established two distinct fusions of PDZ3_ZO-1 and a Trp-cage mini-protein. These fusions with swapped domain order result in FD3A with Trp-cage fused C-terminally and FD4A with Trp-cage fused N-terminally. This study aims to explore the extent to which the distinct Trp-cage fusions affect the function of PDZ3_ZO-1 domain in peptide binding. We find that PDZ3_ZO-1 retains its function, interaction with the connexin 45 peptide, also as part of the fusion proteins. Furthermore, using a phage display approach, we identified a new PDZ3_ZO-1 binding peptide derived from the C-terminal region of methylcytosine dioxygenase TET3. Subsequent validation revealed a significantly higher affinity of PDZ3_ZO-1 for the TET3 peptide as compared to the connexin 45 peptide. Thermodynamic analyses revealed that the swapped domain order conferred distinct effects on the thermodynamic parameters. These results provide insights into the structural and functional adaptability of PDZ domains in engineered proteins, and offer useful principles for the rational design of functional fusion proteins.