The structure of Mycobacterium tuberculosis HtrA reveals an auto-regulatory mechanism
Arvind Kumar Gupta, Debashree Behera and Balasubramanian Gopal [1]
Molecular Tour
There are three HtrA paralogues in M. tuberculosis viz., HtrA (Rv1223), PepD (Rv0983) and PepA (Rv0125). Among these, only HtrA is essential for bacterial survival. M. tuberculosis PepD participates in a two component signaling pathway involving MprAB and σE. It has been suggested that recognition of misfolded proteins by the PDZ domain in PepD activates this pathway. However, unlike DegS, this HtrA homologue is constitutively active and the PDZ domain positively modulates its enzymatic activity. An aspect that is less explored in this context is the role of the N-terminal cytoplasmic domain. Both HtrA and PepD are predicted to have a cytoplasmic polypeptide stretch of 100-150 residues.
A common structural feature of HtrA proteases is that of a trypsin-like serine protease domain attached to one or more PDZ domains. An in silico analysis of the topological arrangement of these proteases suggests that they are likely to adopt a similar Nin-Cout conformation with one transmembrane helix. Given substantial sequence and structural conservation, the precise roles as well as the rationale for multiple HtrA paralogues in a bacterium are difficult to predict. This aspect is of particular significance to M. tuberculosis as HtrA enzymes govern virulence but the molecular details remain unclear.
The crystal structure of M. tuberculosis HtrA (ΔTM HtrA) that was determined at 1.83 Å resolution. We note that this enzyme exhibits both monomeric as well as trimeric forms in solution. The structure reveals a conformation that would require minor structural alterations for proteolytic activity. Structural features thus suggest that M. tuberculosis HtrA is a regulated protease as opposed to the two other paralogues, PepD and PepA. This essential enzyme is thus likely to be involved in specific signal transduction role as opposed to housekeeping in the recognition and degradation of partially folded or misfolded proteins.
The was determined at a resolution of 1.83Å (PDB ID: 6ieo). In this crystal form, there is one molecule of HtrA in the asymmetric unit. The structure of the periplasmic domain reveals one (226-436; colored in royalblue) flexibly tethered to the (443-528; in gold) at the C-terminal end. The (Alpha Helices, Beta Strands , Loops , Turns) referred to as the N-terminal and C-terminal β-barrel. While the N-terminal β-barrel contains the active site residues His270 and Asp306, the C-terminal β-barrel has Ser387 from . The substantial structural conservation across HtrA enzymes suggests a similar reaction mechanism as evident from the positive charge cavity (the oxyanion hole) which helps in stabilization of tetrahedral intermediate during the acylation step of catalysis.
References
- ↑ Gupta AK, Behera D, Gopal B. The crystal structure of Mycobacterium tuberculosis high-temperature requirement A protein reveals an autoregulatory mechanism. Acta Crystallogr F Struct Biol Commun. 2018 Dec 1;74(Pt 12):803-809. doi:, 10.1107/S2053230X18016217. Epub 2018 Nov 29. PMID:30511675 doi:http://dx.doi.org/10.1107/S2053230X18016217
