High-temperature requirement A (HtrA), a highly conserved family of serine protease, plays crucial roles in protein quality control in prokaryotes and eukaryotes. The HtrA protein contains a C-terminal PDZ domain that mediates the proteolytic activity. Here we reported the solution structure of the HtrA PDZ domain from Streptococcus pneumoniae by NMR spectroscopy. Our results showed that the structure of HtrA PDZ domain, which contains three alpha-helices and five beta-strands, illustrates conservation within the canonical PDZ domains. In addition, we demonstrated the interactions between S. pneumoniae HtrA PDZ domain and peptides with the motif XXX-YYF-COOH by surface plasmon resonance. Besides, we identified the ligand binding surface and the critical residues responsible for ligand binding of HtrA PDZ domain by chemical shift perturbation and site-directed mutagenesis. (C) 2011 Elsevier Inc. All rights reserved.