Crystallization and preliminary diffraction analysis of the CAL PDZ domain in complex with a selective peptide inhibitor

A CAL PDZ-domain construct was crystallized in complex with the selective peptide inhibitor iCAL36, which has been shown to enhance ΔF508-CFTR chloride-channel activity in human airway epithelial cells. Cystic fibrosis (CF) is associated with loss-of-function mutations in the CF transmembrane conductance regulator (CFTR), which regulates epithelial fluid and ion homeostasis. The CFTR cytoplasmic C-terminus interacts with a number of PDZ (PSD-95/Dlg/ZO-1) proteins that modulate its intracellular trafficking and chloride-channel activity. Among these, the CFTR-associated ligand (CAL) has a negative effect on apical-membrane expression levels of the most common disease-associated mutant ΔF508-CFTR, making CAL a candidate target for the treatment of CF. A selective peptide inhibitor of the CAL PDZ domain (iCAL36) has recently been developed and shown to stabilize apical expression of ΔF508-CFTR, enhancing net chloride-channel activity, both alone and in combination with the folding corrector corr-4a. As a basis for structural studies of the CAL–iCAL36 interaction, a purification protocol has been developed that increases the oligomeric homogeneity of the protein. Here, the cocrystallization of the complex in space group P2₁2₁2₁, with unit-cell parameters a = 35.9, b = 47.7, c = 97.3 Å, is reported. The crystals diffracted to 1.4 Å resolution. Based on the calculated Matthews coefficient (1.96 ų Da⁻¹), it appears that the asymmetric unit contains two complexes