Functional Characterization of Alr0765, A Hypothetical Protein from Anabaena PCC 7120 Involved in Cellular Energy Status Sensing, Iron Acquisition and Abiotic Stress Management in E. coli Using Molecular, Biochemical and Computational Approaches

Background: Cyanobacteria are excellent model to understand the basic metabolic processes taking place in response to abiotic stress. The present study involves characterization of a hypothetical protein Alr0765 of Anabaena PCC7120 comprising CBS-CP12 domain and deciphering its role in abiotic stress tolerance. Methods: Molecular cloning, heterologous expression and protein purification using affinity chromatography was performed to obtain native purified protein Alr0765. Energy sensing property of Alr0765 was inferred from its binding affinity with different ligand molecules as analyzed by FTIR and TNP-ATP binding assay. AAS and real time-PCR were applied to evaluate the iron acquisition property and cyclic voltammetry was employed to check redox sensitivity of the target protein. Transcript level under different abiotic stresses as well as spot assay, CFU count, ROS level and cellular H2O2 level were used to show potential role of Alr0765 in abiotic stress tolerance. In-silico analysis of Alr0765 included molecular function probability analysis, multiple sequence analysis, protein domain and motif finding, secondary structure analysis, protein ligand interaction, homologous modeling, model refinement and verification and molecular docking was performed with COFACTOR, PROMALS-3D, InterProScan, MEME, TheaDomEx, COACH, Swiss modeller, Modrefiner, PROCHECK, ERRAT, MolProbity, ProSA, TM-align, and Discovery studio respectively. Results: Transcript levels of alr0765 significantly increased by 20, 13, 15, 14.8, 12, 7, 6 and 2.5 fold when Anabaena PCC7120 treated with LC50 dose of heat, arsenic, cadmium, butachlor, salt, mannitol (drought), UV-B, and methyl viologen respectively, with respect to control (untreated). Heterologous expression resulted in 23KDa protein observed on the SDS-PAGE. Immunoblotting and MALDI-TOF-MS/MS followed by MASCOT search analysis confirmed the identity of the protein and ESI/MS revealed the purified protein was a dimer. Binding possibility of Alr0765 with ATP was observed with almost 6-fold increment in relative fluorescence during TNP-ATP binding assay with a ƛ max of 538 nm. FTIR spectra revealed modification in protein confirmation upon binding of Alr0765 with ATP, ADP, AMP and NADH. A 10-fold higher accumulation of iron was observed in digests of E. coli with recombinant vector after induction as compared to control affirms the iron acquisition property of protein. Moreover, generation of redox potential of 146 mV by Alr0765 suggested its probable role in maintaining redox status of the cell under environmental constraints. As per CFU count recombinant E. coli BL21 cells showed about 14.7, 7.3, 6.9, 1.9, 3, 4.9 fold higher number of colonies under heat, cadmium (CdCl2), arsenic (Na3AsO4), salt (NaCl), UV-B and drought (mannitol) respectively compared to pET21a harboring E. coli BL21 cells. Deterioration in cellular ROS level and total cellular H2O2 concentration validated stress tolerance ability of Alr0765. In-silico analysis unraveled novel findings and attested experimental findings in determining the role of Alr0765. Conclusion: Alr0765 is a novel CBS-CP12 domain protein that maintains cellular energy level and iron homeostasis provide tolerance against multiple abiotic stresses.