CtpA from P. aeruginosa, however, behaves differently. At least under the experimental conditions used here, this does not contradict our abovementioned hypothesis of an extracellular localization STA-9090 of C. trachomatis CtpA, but demonstrates that P. aeruginosa CtpA is in fact a periplasmic protease and that the subcellular localization is an important protein characteristic that must be determined to understand

the physiological role of the protein. The same can be said for CTPs from Gram-positive bacteria as bioinformatic analysis of genomic sequence data suggests that these CTPs are secreted to the extracellular environment. CtpA of P. aeruginosa will remain in the periplasm and is not secreted to the extracellular environment or present in the outer membrane. Several dozen reports have been published about bacterial CTPs after the initial study of Hara et Epacadostat cell line al. (1991). Most refer to CTPs as periplasmic proteases, although the experimental evidence

for the individual protein was not given. As far as we know, we are the first to confirm experimentally the exclusive periplasmic localization of a CTP-3 of P. aeruginosa. The periplasmic localization of CtpA strictly excludes the possibility that the protein is directly involved in the virulence of P. aeruginosa as an extracellular effector molecule. The obvious role of CTPs in the virulence of pathogenic bacteria, as shown experimentally in B. suis, B. bacilliformis and B. mallei (Mitchell & Minnick, 1997; Bandara et al., 2005, 2008; Lad et al., 2007) and P. aeruginosa (R. Hoge et al., unpublished data), must be due to an indirect effect mediated by 4��8C a substrate protein of CTP in the context of a periplasmic function. Equivalent to the evolution and function of CTPs from phototrophic organisms, CTPs from Gram-negative

bacteria may be required to activate periplasmatic proteins by cleavage, just as the photosynthetic D1 protein is activated in plant cells. A good candidate as a substrate protein would be the PBP-3. Their periplasmic localization would support evidence of the Prc substrates in E. coli identified by their subcellular localization, because PBP-3 is anchored in the cytoplasmic membrane with the C-terminal end facing the periplasm (Nguyen-Distèche et al., 1998). As PBP-3 in E. coli is involved in the essential process of cell wall synthesis and the CTP could function as an activator of PBP-3, E. coli PBP-3 is thought to be a key element cell division in which it presumably initiates polymerization of the septum peptidoglycan by catalysing a transpeptidation reaction during cell division (Nguyen-Distèche et al., 1998).