Abstract
Omptins constitute a unique family of outer membrane proteases that are widespread in Enterobacteriaceae. The plasminogen activator (Pla) of Yersinia pestis is an omptin family member that is very important for development of both bubonic and pneumonic plague. The physiological function of Pla is to cleave (activate) human plasminogen to form the plasma protease plasmin. Uniquely, lipopolysaccharide (LPS) is essential for the catalytic activity of all omptins, including Pla. Why omptins require LPS for enzymatic activity is unknown. Here, we report the co-crystal structure of LPS-free Pla in complex with the activation loop peptide of human plasminogen, its natural substrate. The structure shows that in the absence of LPS, the peptide substrate binds deep within the active site groove and displaces the nucleophilic water molecule, providing an explanation for the dependence of omptins on LPS for enzymatic activity.
Highlights
Lipids play an important role in membrane protein function
The crystal packing of the complex is such that the residues on the outside of the barrel that are normally involved in LPS binding mediate crystal contacts, making LPS binding physically impossible within the plasminogen activator (Pla)-Alp11 crystal
The structural data presented here provide an explanation for the LPS dependence of omptins for enzymatic activity
Summary
Results: The co-crystal structure of lipid-free Pla protease and a substrate peptide shows that the substrate displaces the active site nucleophilic water molecule. Significance: The structure provides the first explanation for lipid allostery of an outer membrane protein. We report the co-crystal structure of LPS-free Pla in complex with the activation loop peptide of human plasminogen, its natural substrate. The structure shows that in the absence of LPS, the peptide substrate binds deep within the active site groove and displaces the nucleophilic water molecule, providing an explanation for the dependence of omptins on LPS for enzymatic activity. The biological function of membrane proteins can be affected by membrane lipid composition as well as by specific proteinlipid interactions Examples of the latter class are the interaction of cholesterol with G protein-coupled receptors and the activation of the inward rectified Kϩ channel Kir2.2 by phosphatidylinositol 4,5-bisphosphate [2, 3]. The atomic coordinates and structure factors (code 4DCB) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/)
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