| Abstrakt: |
During bacterial cell growth, hydrolases cleave peptide cross-links between strands of the peptidoglycan sacculus to allow new strand insertion. The Pseudomonas aeruginosa carboxyl-terminal processing protease (CTP) CtpA regulates some of these hydrolases by degrading them. CtpA assembles as an inactive hexamer composed of a trimer-of-dimers, but its lipoprotein binding partner LbcA activates CtpA by an unknown mechanism. Here, we report the cryo-EM structures of the CtpA–LbcA complex. LbcA has an N-terminal adaptor domain that binds to CtpA, and a C-terminal superhelical tetratricopeptide repeat domain. One LbcA molecule attaches to each of the three vertices of a CtpA hexamer. LbcA triggers relocation of the CtpA PDZ domain, remodeling of the substrate binding pocket, and realignment of the catalytic residues. Surprisingly, only one CtpA molecule in a CtpA dimer is activated upon LbcA binding. Also, a long loop from one CtpA dimer inserts into a neighboring dimer to facilitate the proteolytic activity. This work has revealed an activation mechanism for a bacterial CTP that is strikingly different from other CTPs that have been characterized structurally. Synopsis: The Pseudomonas aeruginosa cell envelope protease CtpA is activated by the LbcA lipoprotein to degrade cell-wall cross-link hydrolases. Here, structural, biochemical and in vivo approaches show how this activation occurs, revealing a unique mechanism compared to related proteases. CtpA alone is inactive and exists mainly as a dimer at physiological temperature. After LbcA binds to a CtpA dimer, the LbcA-CtpA complex oligomerizes into a CtpA hexamer with three bound LbcA proteins. Only one CtpA molecule is activated within each CtpA dimeric unit. The binding of LbcA triggers a series of conformational changes in the activated CtpA, including a shift of the PDZ domain, realignment of the catalytic site, and remodeling of substrate binding pocket. Structural, biochemical and in vivo approaches reveal a unique mechanism for cell envelope CtpA protease activation by its lipoprotein binding partner LbcA. [ABSTRACT FROM AUTHOR] |
