Lipid Binding of PDZ Domain Containing Proteins and Its Crucial Effect on Cellular Signaling

The PDZ (PSD95, Dlg1, ZO-1) domain is a ubiquitous protein domain that has been considered as a prototypic protein-protein interaction module. Independent groups recently discovered that some PDZ domains have lipid binding affinity; however, it was not known whether the lipid binding activity is a common property of PDZ domains. We thus performed a large sale characterization of PDZ domains and found that about 30% of 70 characterized PDZ domains have strong to intermediate membrane binding activity. We then developed the bioinformatics tools based on our biophysical data and predict the membrane binding property of all PDZ domains. Furthermore, based on the detailed investigation of their membrane binding properties, we classified all lipid binding PDZ domains into three subfamilies. This work shows that membrane binding is a general property of PDZ domains and is important for their physiological functions. It also suggests that other protein-protein interaction domains may also interact with membrane lipids. We understand the physiological significance of the membrane binding of PDZ domains, and we thoroughly investigated the properties of two cytosolic scaffold proteins, NHERF1 and Dvl2 that have PDZ domains with high membrane affinity. The PDZ1 domain of NHERF1 contains a common cholesterol binding motif, known as CRAC motif. Our detailed biophysical and cell studies show that the NHERF1 PDZ1 domain specifically interacts with cholesterol, which is essential for cellular function of NHERF1. The PDZ domain of Dvl2 is also found to bind cholesterol specifically although it lacks the CRAC motif. Our biophysical and biological studies show that cholesterol binding activity of Dvl2 is essential for its role in the canonical Wnt signaling pathway that controls a variety of developmental and homeostatic processes. Most importantly, our studies elucidate the mechanism by which Dvl2 can selectively activate the canonical Wnt signaling over other Wnt signaling pathways. Collectively, these studies provide new insight into how lipids, including cholesterol, can regulate biological processes, including cellular protein-protein interactions.