Bacterial Type I Signal Peptidases as Antibiotic Targets

Smitha Rao CV; Jozef Anné

Disclosures

Future Microbiol. 2011;6(11):1279-1296. 

In This Article

What Makes the Bacterial SPase an Interesting Target?

SPases fulfill the basic criteria required for a drug target and are attractive[10,16] for reasons outlined below:

  • SPases are ubiquitous as they are present in nearly all known bacteria, except for Mycoplasma genitalium.[17,18] Interestingly, SPase-like activity has been demonstrated in the related organism Mycoplasma pneumoniae despite the absence of a gene encoding SPase;[19]

  • SPase activity is essential for cell growth and viability[20,21] as inhibition of SPase activity leads to accumulation of secretory proteins in the cell membrane and eventual cell death.[22] Temperature-sensitive Escherichia coli strains IT89/IT41[23] with an amber mutation in the lepB gene[24] show accumulation of preproteins and lack of growth at the nonpermissive temperature of 42°C. These temperature-sensitive mutant strains have been used to confirm the essential function of SPases from several bacteria including human pathogens such as S. aureus,[24]S. epidermidis,[25]Streptococcus pneumoniae,[26]Salmonella Typhimurium,[27]Legionella pneumophila,[28]Rickettsia rickettsii and Rickettsia typhi;[29]

  • Targeting the bacterial SPases is likely to have limited toxicity to humans because of their difference in structure, localization and suggested catalytic mechanism compared with the eukaryotic signal peptidases. The bacterial SPases are monomeric, and are located at the cytoplasmic membrane surface with their active site on the outer leaflet, making it relatively accessible to potential inhibitors. They operate by a Ser/Lys catalytic dyad mechanism. The eukaryotic signal peptidases are multimeric, and have their active site region located in the mitochondrial intermembrane space (in case of the mitochondrial inner membrane peptidase [Imp], Imp1 and Imp2 subunits) or in the lumen (ER SPase complex). ER SPases do not use Lys as a general base, whereas the mitochondrial SPases, Imp1 and Imp2 use the Ser/Lys catalytic dyad, similar to the bacterial SPases.[15] The bacterial SPases also differ from the classical serine proteases (utilizing the Ser/His/Asp catalytic triad mechanism) in that they attack the peptide backbone of the substrate from the si-face, rather than from the re-face, further enabling selective inhibition.[30] While the common catalytic mechanism for bacterial SPases and mitochondrial SPases is a cause for concern, the location of the mitochondrial SPases means that the inhibitor will have to pass not only through the selectively permeable cytoplasmic membrane, but also through the tightly controlled mitochondrial membrane barriers before it is in a position to cause any unwanted side effects. In addition, the mitochondrial SPases differ in their substrate specificity from their bacterial counterparts, which possibly reflects a difference in the substrate-binding pockets of the enzymes (see review[15]). As the crystal structure of the mitochondrial SPase is not known yet, it remains to be seen whether these minor structural differences can be exploited to improve selective inhibition, thereby minimizing damage to the host cell. However, intracellular pathogens such as Salmonella, mycobacteria and Legionella, which survive in vacuolar niches,[31] present an interesting challenge as the antibiotic needs to penetrate into the pathogen-containing vacuole without affecting the mitochondrial SPase.

  • Targeting the SPases impedes some bacterial defenses, thereby increasing its susceptibility to antibiotics or sensitizing it to other antibiotics. SPase plays an indirect but important role in bacterial defense as its native substrates include β-lactamases that confer resistance to β-lactam antibiotics, toxins (e.g., pertussis toxin from Bordetella pertussis), adhesins (e.g., HMW1 of H. influenzae), proteases (e.g., IgA protease from Neisseria gonorrhoeae) and virulence factors. In Listeria monocytogenes, a food-borne pathogen, deletion of one of the three SPase-encoding genes, sipZ, impaired secretion of virulence factors, such as listeriolysin O and phospholipase C, and rendered it almost avirulent.[32] In Chlamydia trachomatis, an intracellular pathogen, a virulence factor called the chlamydial proteasome/protease-like activity factor (CPAF), which is secreted into the host cell cytosol for degrading various host factors and benefiting intracellular survival, is completely blocked by the SPase inhibitor, arylomycin.[33] The secretion of multiple virulence factors in a nosocomial pathogen, S. epidermidis, requires SPase activity, as demonstrated using the SPase inhibitor.[34] In addition, the S. aureus SPase, SpsB also plays a role in agr quorum sensing system, which is important for biofilm formation and virulence.[35] Taken together, the bacterial dependence on SPase for processing proteins necessary for biofilm growth, virulence, damaging the host tissues, conferring antibiotic resistance and quorum sensing, enhances the significance of finding novel SPase inhibitors.

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