N‐terminal processing of membrane‐targeted MnSOD and formation of multiple active superoxide dismutase dimers in the nitrogen‐fixing cyanobacterium Anabaena sp. strain PCC7120

• Published in: The FEBS journal 2013-10, Vol.280 (19), p.4827-4838
• Main Authors: Raghavan, Prashanth S., Rajaram, Hema, Apte, Shree K.
• Format: Article
• Language: English
• Subjects: Anabaena; Anabaena - enzymology; Bacteria; Cyanobacteria; Membranes; membrane‐targeted protein processing; MnSOD; nitrogen fixation; Oxidation; Oxidative Stress; Protein Multimerization; Strain; Superoxide Dismutase - chemistry; Superoxide Dismutase - metabolism
• ISSN: 1742-464X; 1742-4658
• DOI: 10.1111/febs.12455

Anabaena sp. strain PCC7120 expresses a 30 kDa manganese‐dependent superoxide dismutase (MnSOD) comprising a hydrophobic region (signal peptide + linker peptide) attached to a catalytic unit. Bioinformatics predicted cleavage of the signal peptide at 25CQPQ by signal peptidase and of the linker peptide by an Arg‐C‐like protease at the Arg52/Arg59 residue. The three predicted forms of MnSOD were immunodetected in Anabaena, with the 30 kDa MnSOD found exclusively in the membrane and the shorter 27 and 24 kDa forms found both in the membrane and soluble fractions. The corresponding sodA gene was truncated for (a) the first eight residues, or, (b) the signal peptide, or (c) the entire hydrophobic region, or (d) the Arg52/Arg59 residues were modified to serine. Overexpression of these MnSOD variants in recombinant Anabaena strains revealed that (a) the 30 kDa membrane‐targeted MnSOD was cleaved by membrane‐localized signal peptidase either during or after its transport through the membrane to release the 27 kDa form, either in the cytosol or in the periplasmic/thylakoid lumen, (b) the 27 kDa form was further cleaved to the 24 kDa form by Arg‐C‐like protease, both in the cytosol and in the periplasmic/thylakoid lumen, (c) deletion of signal peptide localized the MnSOD forms in the cytosol, and (d) alteration of the signal/linker peptide cleavage sites interfered with MnSOD localization and processing. Homo/heterodimerization of the 24 and 27 kDa forms of MnSOD and the cytosolic iron‐dependent SOD results in multiple SOD activities, from a single MnSOD gene (sodA), in different cellular compartments of Anabaena. The membrane‐targeted 30 kDa Anabaena MnSOD is cleaved by a signal peptidase, either during or after translocation across the membrane, to release a 27 kDa form in the cytosol or periplasm/thylakoid lumen respectively. Arg‐C‐like proteases further process it to its 24 kDa catalytic unit. The 24 kDa/27 kDa forms of MnSOD and FeSOD interact, to generate several active SOD homo/heterodimers.