Differential recognition of β-catenin and TRIP6 by MAGI1 PDZ6 and allosteric modulation by DLL1.

MAGI1 is a multidomain scaffold protein that coordinates signaling complexes involved in cell adhesion, polarity, and signal transduction. While PDZ domains are generally regarded as independent interaction modules, the role of interdomain communication in regulating partner recognition remains poorly understood. Here, we investigated the molecular basis of partner recognition by its sixth PDZ domain (PDZ6) and the contribution of neighboring domains to this process. Using stopped-flow kinetics, we characterized the interactions of PDZ6 with two physiological partners, β-catenin and TRIP6. The two proteins displayed distinct kinetic and thermodynamic behaviors, indicating different recognition mechanisms despite targeting the same domain. Analysis of ionic strength and pH dependence revealed distinct molecular determinants underlying the two interactions. While β-catenin binding was largely insensitive to the multidomain context, TRIP6 recognition was selectively influenced by adjacent domains. Remarkably, binding of DLL1 to the neighboring PDZ5 domain strongly enhanced TRIP6 affinity while exerting minimal effects on β-catenin, and this effect was further modulated by pH. Together, these findings support a model in which MAGI1 behaves as a dynamically coupled scaffold whose binding properties are shaped by interdomain communication, highlighting the importance of studying PDZ-mediated interactions within their multidomain context.