Elongation of the Poly-γ-glutamate Tail of F420 Requires Both Domains of the F420:γ-Glutamyl Ligase (FbiB) of Mycobacterium tuberculosis

Ghader Bashiri,Aisyah M Rehan,Sreevalsan Sreebhavan,Heather M Baker,Edward N Baker,Christopher J Squire

doi:10.1074/jbc.m115.689026

Ghader Bashiri, Aisyah M Rehan + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m115.689026

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Abstract

Cofactor F420is an electron carrier with a major role in the oxidoreductive reactions ofMycobacterium tuberculosis, the causative agent of tuberculosis. A γ-glutamyl ligase catalyzes the final steps of the F420biosynthesis pathway by successive additions ofl-glutamate residues to F420-0, producing a poly-γ-glutamate tail. The enzyme responsible for this reaction in archaea (CofE) comprises a single domain and produces F420-2 as the major species. The homologousM. tuberculosisenzyme, FbiB, is a two-domain protein and produces F420with predominantly 5-7l-glutamate residues in the poly-γ-glutamate tail. The N-terminal domain of FbiB is homologous to CofE with an annotated γ-glutamyl ligase activity, whereas the C-terminal domain has sequence similarity to an FMN-dependent family of nitroreductase enzymes. Here we demonstrate that full-length FbiB adds multiplel-glutamate residues to F420-0in vitroto produce F420-5 after 24 h; communication between the two domains is critical for full γ-glutamyl ligase activity. We also present crystal structures of the C-terminal domain of FbiB in apo-, F420-0-, and FMN-bound states, displaying distinct sites for F420-0 and FMN ligands that partially overlap. Finally, we discuss the features of a full-length structural model produced by small angle x-ray scattering and its implications for the role of N- and C-terminal domains in catalysis.

Highlights

The cofactor F420 is a flavin derivative that is sporadically distributed among microorganisms, mainly archaea and actinobacteria
Functional Characterization—The FbiB constructs expressed in E. coli were used for functional studies to alleviate the complications arising from the presence of co-purified F420 in the proteins expressed in M. smegmatis (Fig. 1B)
The F420-0 substrate was prepared by enzymatic hydrolysis of the poly-␥-glutamate tail of F420 using carboxypeptidase G [17], and the resulting F420-0 was used as a substrate for FbiB activity experiments and for crystallographic binding studies

Summary

Experimental Procedures

The resulting entry clones were used to clone the full-length construct into pDEST17 [21] and pDESTsmg [22] vectors using an LR reaction. The expression construct for pDEST17 was selected on LB agar plates containing 100 ␮g/ml ampicillin. Expression and Purification—FbiB constructs were expressed in Escherichia coli BL21(DE3)pRP and Mycobacterium smegmatis mc2 4517 [23] cells In both cases, protein expression was performed in autoinduction medium as described previously [24]. The His6-tagged rTEV protease, encoded in a pProEX HTa expression vector, was produced earlier from E. coli RosettaTM(DE3)pLysS cells (Novagen), as described previously [25]. The binding reactions contained 20 mM HEPES, pH 7.0, 150 mM NaCl, 1 mM ␤-mercaptoethanol and were corrected against a control lacking the FbiB protein. Activity Assays—The ␥-glutamyl ligase activity [17] was measured in 50-␮l reactions containing different FbiB constructs

Unique reflectionsa

Root mean square deviation from ideality

Results

Discussion