The AAX system from Chlamydia pneumoniae
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Arginine uptake and degradation systems are common throughout bacteria and archaea. The genome of human pathogen Chlamydia pneumoniae encodes three proteins now called AaxA, AaxB, and AaxC which function together to take up arginine, decarboxylate it, and expel the decarboxylation product, agmatine. AaxB is the previously characterized pyruvoyl-dependent arginine decarboxylase, AaxC is an inner membrane amino acid transport protein that functions as an arginine-agmatine antiporter, and AaxA is an outer membrane porin, which facilitates the uptake of arginine and also functions as a general porin with broad specificity. C. pneumoniae is a non-typical Gram negative bacteria and an obligate intracellular parasite with a unique 2-phase life cycle. The role of this system for arginine-agmatine exchange has yet to be determined but it may function to deplete host cell arginine as a means of inactivating host inducible nitric oxide synthase (iNOS), a molecule used in the innate immune response that has been shown to have an inhibitory affect on the growth of C. pneumoniae in cell culture. AaxB and AaxC are able to complement the loss of extreme acid-resistance in E. coli mutants that lack their own system for arginine-agmatine exchange, making pH homeostasis another possible role for this system. The porin AaxA is able to enhance arginine-agmatine exchange by AaxB and AaxC in E. coli mutants as well as by the native arginine decarboxylase AdiA and the native arginine-agmatine antiporter AdiC in wild type E. coli. AaxA is not an arginine-specific porin and instead acts as a general porin with a broad specificity. AaxA discriminates only against large and negatively charged solute molecules, and therefore it may have a broad role in the uptake of various biomolecules essential for chlamydial growth in addition to its role as part of a system for arginine-agmatine exchange.