N-terminal processing of membrane-targeted MnSOD andformation of multiple active superoxide dismutase dimersin the nitrogen-fixing cyanobacterium Anabaena sp. strainPCC7120

摘要

Anabaena sp. strain PCC7120 expresses a 30 kDa manganese-dependentsuperoxide dismutase (MnSOD) comprising a hydrophobic region (signalpeptide + linker peptide) attached to a catalytic unit. Bioinformatics predictedcleavage of the signal peptide at 25CQPQ by signal peptidase and ofthe 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 theshorter 27 and 24 kDa forms found both in the membrane and solublefractions. The corresponding sodA gene was truncated for (a) the first eightresidues, or, (b) the signal peptide, or (c) the entire hydrophobic region, or(d) the Arg52/Arg59 residues were modified to serine. Overexpression ofthese MnSOD variants in recombinant Anabaena strains revealed that (a)the 30 kDa membrane-targeted MnSOD was cleaved by membrane-localizedsignal peptidase either during or after its transport through the membraneto release the 27 kDa form, either in the cytosol or in theperiplasmic/thylakoid lumen, (b) the 27 kDa form was further cleaved tothe 24 kDa form by Arg-C-like protease, both in the cytosol and in theperiplasmic/thylakoid lumen, (c) deletion of signal peptide localized theMnSOD forms in the cytosol, and (d) alteration of the signal/linker peptidecleavage sites interfered with MnSOD localization and processing. Homo/heterodimerization of the 24 and 27 kDa forms of MnSOD and the cytosoliciron-dependent SOD results in multiple SOD activities, from a singleMnSOD gene (sodA), in different cellular compartments of Anabaena.