The binding affinity of PTPN13's tandem PDZ2/3 domain is allosterically modulated

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Cryptic binding properties of a transient folding intermediate in a PDZ tandem repeat

PDZ domains are the most diffused protein-protein interaction modules of the human proteome and are often present in tandem repeats. An example is PDZD2, a protein characterized by the presence of six PDZ domains that undergoes a proteolytic cleavage producing sPDZD2, comprising a tandem of two PDZ domains, namely PDZ5 and PDZ6. Albeit the physiopathological importance of sPDZD2 is well-established, the interaction with endogenous ligands has been poorly characterized. To understand the determinants of the stability and function of sPDZD2, we investigated its folding pathway. Our data highlights the presence of a complex scenario involving a transiently populated folding intermediate that may be accumulated from the concurrent denaturation of both PDZ5 and PDZ6 domains. Importantly, double jump kinetic experiments allowed us to pinpoint the ability of this transient intermediate to bind the physiological ligand of sPDZD2 with increased affinity compared to the native state. In summary, our results provide an interesting example of a functionally competent misfolded intermediate, which may exert a cryptic function that is not captured from the analysis of the native state only.

Conformational flexibility of GRASPs and their constituent PDZ subdomains reveals structural basis of their promiscuous interactome

The Golgi complex is a central component of the secretory pathway, responsible for several critical cellular functions in eukaryotes. The complex is organized by the Golgi matrix that includes the Golgi reassembly and stacking protein (GRASP), which was shown to be involved in cisternae stacking and lateral linkage in metazoan. GRASPs also have critical roles in other processes, with an unusual ability to interact with several different binding partners. The conserved N terminus of the GRASP family includes two PSD-95, DLG, and ZO-1 (PDZ) domains. Previous crystallographic studies of orthologues suggest that PDZ1 and PDZ2 have similar conformations and secondary structure content. However, PDZ1 alone mediates nearly all interactions between GRASPs and their partners. In this work, NMR, synchrotron radiation CD, and molecular dynamics (MD) were used to examine the structure, flexibility, and stability of the two constituent PDZ domains. GRASP PDZs are structured in an unusual β₃ α₁ β₄ β₅ α₂ β₆ β₁ β₂ secondary structural arrangement and NMR data indicate that the PDZ1 binding pocket is formed by a stable β₂ -strand and a more flexible and unstable α₂ -helix, suggesting an explanation for the higher PDZ1 promiscuity. The conformational free energy profiles of the two PDZ domains were calculated using MD simulations. The data suggest that, after binding, the protein partner significantly reduces the conformational space that GRASPs can access by stabilizing one particular conformation, in a partner-dependent fashion. The structural flexibility of PDZ1, modulated by PDZ2, and the coupled, coordinated movement between the two PDZs enable GRASPs to interact with multiple partners, allowing them to function as promiscuous, multitasking proteins.

Scribble co-operatively binds multiple α1D-adrenergic receptor C-terminal PDZ ligands

Many G protein-coupled receptors (GPCRs) are organized as dynamic macromolecular complexes in human cells. Unraveling the structural determinants of unique GPCR complexes may identify unique protein:protein interfaces to be exploited for drug development. We previously reported α_1D-adrenergic receptors (α_1D-ARs) – key regulators of cardiovascular and central nervous system function – form homodimeric, modular PDZ protein complexes with cell-type specificity. Towards mapping α_1D-AR complex architecture, biolayer interferometry (BLI) revealed the α_1D-AR C-terminal PDZ ligand selectively binds the PDZ protein scribble (SCRIB) with >8x higher affinity than known interactors syntrophin, CASK and DLG1. Complementary in situ and in vitro assays revealed SCRIB PDZ domains 1 and 4 to be high affinity α_1D-AR PDZ ligand interaction sites. SNAP-GST pull-down assays demonstrate SCRIB binds multiple α_1D-AR PDZ ligands via a co-operative mechanism. Structure-function analyses pinpoint R1110^PDZ4 as a unique, critical residue dictating SCRIB:α_1D-AR binding specificity. The crystal structure of SCRIB PDZ4 R1110G predicts spatial shifts in the SCRIB PDZ4 carboxylate binding loop dictate α_1D-AR binding specificity. Thus, the findings herein identify SCRIB PDZ domains 1 and 4 as high affinity α_1D-AR interaction sites, and potential drug targets to treat diseases associated with aberrant α_1D-AR signaling.