Nuclear Magnetic Resonance Characterization of the Type III Secretion System Tip Chaperone Protein PcrG of <i>Pseudomonas aeruginosa</i>

Lung infection with <i>Pseudomonas aeruginosa</i> is the leading cause of death among cystic fibrosis patients. To initiate infection, <i>P. aeruginosa</i> assembles a protein nanomachine, the type III secretion system (T3SS), to inject bacterial proteins directly into target host cells. An important regulator of the <i>P. aeruginosa</i> T3SS is the chaperone protein PcrG, which forms a complex with the tip protein, PcrV. In addition to its role as a chaperone to the tip protein, PcrG also regulates protein secretion. PcrG homologues are also important in the T3SS of other pathogens such as <i>Yersinia pestis</i>, the causative agent of bubonic plague. The atomic structure of PcrG or any member of the family of tip protein chaperones is currently unknown. Here, we show by circular dichroism and nuclear magnetic resonance (NMR) spectroscopy that PcrG lacks a tertiary structure. However, it is not completely disordered but contains secondary structures dominated by two long α-helices from residue 16 to 41 and from residue 55 to 76. The helices of PcrG are partially formed, have similar backbone dynamics, and are flexible. NMR titrations show that the entire length of PcrG residues from position 9 to 76 is involved in binding to PcrV. PcrG adds to the growing list of partially folded or unstructured proteins with important roles in type III secretion.