Mycobacterium tuberculosis ArfA Rv0899

Function

Protein Rv0899 from Mycobacterium tuberculosis [1] belongs to the OmpA (outer membrane protein A) family of outer membrane proteins.The deletion of this gene impairs the uptake of some water-soluble substances, such as serine, glucose, and glycerol. It was proposed that Rv0899 forms an oligomeric channel to carry out such functions.Using NMR chemical shift perturbation and isothermal calorimetric titration assays, Rv0899 was able to interact with Zn(2+) ions, which may indicate a role for Rv0899 in the process of Zn(2+) acquisition. ^[1] Rv0899 has been proposed to act as an outer membrane porin and to contribute to the bacterium's adaptation to the acidic environment of the phagosome[2] during infection. The gene is restricted to pathogenic mycobacteria and, thus, is an attractive candidate for the development of anti-tuberculosis chemotherapy. The 326-residue protein contains three domains: an N-terminal domain (residues 1-72) that includes a sequence of 20 hydrophobic amino acids required for membrane translocation, a central B domain (residues 73-200) with homology to the conserved putative lipid-binding BON (bacterial OsmY and nodulation) superfamily[3] , and a C domain (residues 201-326) with homology to the OmpA-C-like superfamily of periplasmic peptidoglycan-binding sequences, found in several types of bacterial membrane proteins, including in the C-terminus of the Escherichia coli outer membrane protein OmpA. Rv0899 does not form a transmembrane beta-barrel. Residues 73-326 form a mixed alpha/beta-globular structure, encompassing two independently folded modules corresponding to the B and C domains connected by a flexible linker. The B domain folds with three parallel/antiparallel alpha-helices packed against six parallel/antiparallel beta-strands that form a flat beta-sheet. The core is hydrophobic, while the exterior is polar and predominantly acidic.^[2] Mycobacterium tuberculosis ArfA (Rv0899) is a membrane protein encoded by an operon that is required for supporting bacterial growth in acidic environments. Its C-terminal domain (C domain) shares significant sequence homology with the OmpA-like family of peptidoglycan-binding domains, suggesting that its physiological function in acid stress protection may be related to its interaction with the mycobacterial cell wall. ArfA is identified as a peptidoglycan-binding protein. The C domain of ArfA adopts the characteristic fold of the OmpA-like family. It exhibits pH-dependent conformational dynamics (with significant heterogeneity at neutral pH and a more ordered structure at acidic pH), which could be related to its acid stress response. The C domain associates tightly with polymeric peptidoglycan isolated from M. tuberculosis and also associates with a soluble peptide intermediate of peptidoglycan biosynthesis. This enabled us to characterize the peptidoglycan binding site where five highly conserved ArfA residues, including two key arginines, establish the specificity for diaminopimelate- but not Lys-type peptidoglycan. ArfA is the first peptidoglycan-binding protein to be identified in M. tuberculosis. Its functions in acid stress protection and peptidoglycan binding suggest a link between the acid stress response and the physicochemical properties of the mycobacterial cell wall.^[3] OmpATb is not a general porin.Chemical analysis of low-pH culture filtrates showed that the proteins encoded by the ompATb operon are involved in generating a rapid ammonia burst, which neutralized medium pH Addition of ammonia accelerated growth of the ompATb operon mutant demonstrating that ammonia secretion is indeed a mechanism by which M. tuberculosis neutralizes acidic environments. Infection experiments revealed that the ompATb operon was not required for full virulence in mice suggesting that M. tuberculosis has multiple mechanisms of resisting phagosomal acidification. The ompATb operon is necessary for rapid ammonia secretion and adaptation of M. tuberculosis to acidic environments in vitro but not in mice. ^[4]

Disease

Probably plays a role in ammonia secretion that neutralizes the medium at pH 5.5,and preceded exponential growth of M. tuberculosis, although it does not play a direct role in ammonia transport.[ARFA_MYCTU]. Its functions in acid stress protection and peptidoglycan binding suggest a link between the acid stress response and the physicochemical properties of the mycobacterial cell wall.^[5]

Relevance

Structural highlights