New Perspectives on BolA: A Still Mysterious Protein Connecting Morphogenesis, Biofilm Production, Virulence, Iron Metabolism, and Stress Survival

Abstract

The BolA-like protein family is widespread among prokaryotes and eukaryotes. BolA was originally described in E. coli as a gene induced in the stationary phase and in stress conditions. The BolA overexpression makes cells spherical. It was characterized as a transcription factor modulating cellular processes such as cell permeability, biofilm production, motility, and flagella assembly. BolA is important in the switch between motile and sedentary lifestyles having connections with the signaling molecule c-di-GMP. BolA was considered a virulence factor in pathogens such as Salmonella Typhimurium and Klebsiella pneumoniae and it promotes bacterial survival when facing stresses due to host defenses. In E. coli, the BolA homologue IbaG is associated with resistance to acidic stress, and in Vibrio cholerae, IbaG is important for animal cell colonization. Recently, it was demonstrated that BolA is phosphorylated and this modification is important for the stability/turnover of BolA and its activity as a transcription factor. The results indicate that there is a physical interaction between BolA-like proteins and the CGFS-type Grx proteins during the biogenesis of Fe-S clusters, iron trafficking and storage. We also review recent progress regarding the cellular and molecular mechanisms by which BolA/Grx protein complexes are involved in the regulation of iron homeostasis in eukaryotes and prokaryotes.

Keywords: BolA-like proteins; Fe-S proteins; IbaG; biofilm; bolA; c-di-GMP; flagella; glutaredoxin; phosphorylation; virulence.

Figure 1

Maximum-likelihood phylogenetic reconstructions of bolA1 using MEGA software v11.0.11 [13]. Distance estimation was obtained by Tamura–Nei model. Numbers at the nodes represent bootstrap values (%) based on 1000 replicates. Gene sequences were aligned using MUSCLE and the trees were reconstructed using default settings. The sequences of each gene were obtained from NCBI database [12] and the GenBank accession numbers are written next to each species.

Figure 2

Phylogenetic analysis of BolA-like proteins. ClustalW multiple sequence alignment of E. coli BolA protein, used as template, with BolA-like proteins from prokaryotes and some eukaryotic organisms. Identical residues are in gray and black. The putative E. coli phosphorylated residues, are highly conserved and colored in blue. The only cysteine present in E. coli BolA is also highly conserved and is colored in rose. The * represents the phosphorylated sites in E. coli. The secondary αββαβ structure was predicted in Net Secondary Structure Prediction Jalview. The secondary structure, regarding E. coli BolA protein, is shown above the alignment. The sequence ID for the proteins is presented: Ec: E. coli-WP_024139872.1; Cb: Coxiella burnetii- AAO90126.1; Bv: Babesia bovis-XP_001609842.1; Lm: Listeria monocytogenes-WP_024139872.1; Al: Agitococcus lubricus-WP_107866244.1; Pp: Pseudomonas putida-WP_132844560.1; Cj: Chromohalobacter japonicus-WP_040243100.1; Hi: Haemophilus influenzae-WP_015701425.1; Ba: Buchnera aphidicola-WP_009874425.1; Au: Aerococcus urinaeWP_111852928.1; Sc: Saccharomyces cerevisiae-NP_075206.1; Dd: Dictyostelium discoideumXP_644296.1; Cr: Chlamydomonas reinhardtii- XP_001702905.1; At: Arabidopsis thaliana 1-BAD43461.1; At: Arabidopsis thaliana 2-Q84W65; Bt: Bos taurus-NP_001029524.1; Pa: Pongo abelli NP_001125298.1; Mm: Mus musculus-NP_081251.1; Hs: Homo sapiens-NP_001307954.1 [45,46].

Figure 3

Pleiotropic effects of E. coli BolA. In green are represented features/mechanisms repressed by BolA. Mechanisms promoted by BolA can be seen in red. Blue arrows represent the role of BolA in iron homeostasis, a pathway that is still not well understood. The cartoon in the center represents superposition of BolA NMR structure of three BolA proteins (B): E. coli (PDB ID 2DHM, red): C. burnetti (PDB ID 3TR3, green), B. bovis (PDB ID 3O2E, blue) [14].

Figure 4

Schematic diagram of FeS cluster transfer/maturation in E. coli. In E. coli the synthesis of FeS clusters is catalyzed by complex machinery that includes ISC system. [2Fe-2S] clusters are distributed in a process that depends on the Glutathione-S-transferase (GSH, colored in yellow), CGFS-type GrxD homodimers (colored in green) and GrxD-BolA heterocomplexes, (BolA colored in blue). The reactions are reversible and monomeric BolAs or GrxD-BolA could be reformed. The physiological relevance of each form has not been elucidated but this interconversion could play a role in FeS trafficking and homeostasis.