Tripodal Lipoprotein Variants with C-Terminal Hydrophobic Residues Allosterically Modulate Activity of the DegP Protease

Abstract

DegP, a member of the highly conserved HtrA family of proteases, performs a regulated proteolysis of toxic misfolded proteins in the periplasm of Gram-negative bacteria. The allosteric switch between inactive and active conformations is a central mechanism to carefully control proteolytic activity of DegP and to maintain the optimal cellular fitness, but few other molecules than substrates are known to allosterically control DegP activity. Here, we demonstrate that a mutant variant of an outer-membrane lipoprotein, Lpp^+Leu, can function as a novel allosteric effector that changes the dynamic range of DegP activity. The three leucines at the C-termini of trimeric Lpp^+Leu are central components for activity modulation. Selection experiments with Lpp variant libraries show that Lpp variants with diverse sequences at or near C-termini, in particular those with hydrophobic residues at C-termini, function similarly to Lpp^+Leu. Interestingly, Lpp variants carrying different residues at C-terminal, penultimate, or antepenultimate positions display dramatically different patterns of activation and inhibition effects, suggesting that their interactions with DegP differentially stabilize distinct inactive or active conformations. We propose that the tripodal structure with three hydrophobic ends that mimics Lpp^+Leu is a novel platform for allosteric effectors, which may be useful in developing new antibiotics against DegP or homologous HtrA proteases.

Keywords: HtrA; Lpp; allosteric effectors; protein quality control; regulated proteolysis.