Mycobacterium tuberculosis Hip1 modulates macrophage responses through proteolysis of GroEL2

Mycobacterium tuberculosis (Mtb) employs multiple strategies to evade host immune responses and persist within macrophages. We have previously shown that the cell envelope-associated Mtb serine hydrolase, Hip1, prevents robust macrophage activation and dampens host pro-inflammatory responses, allowing Mtb to delay immune detection and accelerate disease progression. We now provide key mechanistic insights into the molecular and biochemical basis of Hip1 function. We establish that Hip1 is a serine protease with activity against protein and peptide substrates. Further, we show that the Mtb GroEL2 protein is a direct substrate of Hip1 protease activity. Cleavage of GroEL2 is specifically inhibited by serine protease inhibitors. We mapped the cleavage site within the N-terminus of GroEL2 and confirmed that this site is required for proteolysis of GroEL2 during Mtb growth. Interestingly, we discovered that Hip1-mediated cleavage of GroEL2 converts the protein from a multimeric to a monomeric form. Moreover, ectopic expression of cleaved GroEL2 monomers into the hip1 mutant complemented the hyperinflammatory phenotype of the hip1 mutant and restored wild type levels of cytokine responses in infected macrophages. Our studies point to Hip1-dependent proteolysis as a novel regulatory mechanism that helps Mtb respond rapidly to changing host immune environments during infection. These findings position Hip1 as an attractive target for inhibition for developing immunomodulatory therapeutics against Mtb.

Human cytochrome b5 requires residues E48 and E49 to stimulate the 17,20-lyase activity of cytochrome P450c17

Cytochrome P450c17 (CYP17) catalyzes both the 17alpha-hydroxylase and 17,20-lyase reactions in human steroid biosynthesis. Cytochrome b5 (b5) stimulates the rate of the 17,20-lyase reaction 10-fold with little influence on 17alpha-hydroxylase activity. Studies with apo-b5 suggest that stimulation of 17,20-lyase activity results from an allosteric action on the hCYP17 x POR complex, rather than electron transfer by b5. We hypothesized that specific residues on b5 interact with the hCYP17 x POR complex and that targeted mutation of surface-exposed residues might identify b5 residues critical for stimulating 17,20-lyase activity. We constructed, expressed, and purified 14 single plus 3 double b5 mutations and assayed their ability to stimulate 17,20-lyase activity. Most mutations did not alter the capacity of b5 to stimulate 17,20-lyase activity or appeared to modestly alter the affinity of b5 for the hCYP17 x POR complex. In contrast, mutation of E48, E49, or R52 reduced the maximal stimulation of 17,20-lyase activity. In particular, b5 mutation E48G + E49G lost over 95% of the capacity to stimulate 17,20-lyase activity, yet this mutation retained normal electron transfer properties. In addition, mutation E48G + E49G did not impair stimulation of 17,20-lyase activity by wild-type b5, suggesting that the mutation binds poorly to the site of the hCYP17 x POR complex occupied by b5. These data suggest that a specific allosteric binding site on b5, which includes residues E48, E49, and possibly R52, mediates the stimulation of 17,20-lyase activity.