Unique N-terminal Arm of Mycobacterium tuberculosis PhoP Protein Plays an Unusual Role in Its Regulatory Function

PhoP impacts numerous aspects of M. tuberculosis physiology. However, the mechanism of activation of its C-terminal effector domain remains unknown. Results: PhoP contains a functionally essential and unique N-terminal arm. Conclusion: Conformational change of the arm-truncated variant of PhoP accounts for its loss of function. Significance: The presence of the arm of PhoP helps explain the complexity of regulatory functions of the protein in M. tuberculosis. Mycobacterium tuberculosis PhoP, a master regulator involved in complex lipid biosynthesis and expression of unknown virulence determinants, is composed of an N-terminal receiver domain and a C-terminal effector domain. The two experimentally characterized PhoP orthologs, from Escherichia coli and Salmonella enterica, display vastly different regulatory capabilities. Here, we demonstrate that the 20-residue-long N-terminal arm unique to M. tuberculosis PhoP plays an essential role in the expanded regulatory capabilities of this important regulator. Although the arm is not required for overall structural stability and/or phosphorylation of the PhoP N-domain, strikingly it is essential for phosphorylation-coupled transcription regulation of target genes. Consistent with this view, arm truncation of PhoP is accompanied by a conformational change of the effector domain, presenting a block in activation subsequent to phosphorylation. These results suggest that presence of the arm, unique to this regulator that shares an otherwise highly conserved domain structure with members of the protein family, contributes to the mechanism of inter-domain interactions. Thus, we propose that the N-terminal arm is an adaptable structural feature of M. tuberculosis PhoP, which evolved to fine-tune regulatory capabilities of the transcription factor in response to the changing physiology of the bacilli within its host.