title: Functional Differentiation of Antiporter-Like Polypeptides in Complex I; a Site-Directed Mutagenesis Study of Residues Conserved in MrpA and NuoL but Not in MrpD, NuoM, and NuoN.
abstract: It has long been known that the three largest subunits in the membrane domain (NuoL, NuoM and NuoN) of complex I are homologous to each other, as well as to two subunits (MrpA and MrpD) from a Na+/H+ antiporter, Mrp. MrpA and NuoL are more similar to each other and the same is true for MrpD and NuoN. This suggests a functional differentiation which was proven experimentally in a deletion strain model system, where NuoL could restore the loss of MrpA, but not that of MrpD and vice versa. The simplest explanation for these observations was that the MrpA and MrpD proteins are not antiporters, but rather single subunit ion channels that together form an antiporter. In this work our focus was on a set of amino acid residues in helix VIII, which are only conserved in NuoL and MrpA (but not in any of the other antiporter-like subunits.) and to compare their effect on the function of these two proteins. By combining complementation studies in B. subtilis and 23Na-NMR, response of mutants to high sodium levels were tested. All of the mutants were able to cope with high salt levels; however, all but one mutation (M258I/M225I) showed differences in the efficiency of cell growth and sodium efflux. Our findings showed that, although very similar in sequence, NuoL and MrpA seem to differ on the functional level. Nonetheless the studied mutations gave rise to interesting phenotypes which are of interest in complex I research.
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Eva
authors.initials: E
authors.lastname: Sperling
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Kamil
authors.initials: K
authors.lastname: Górecki
authors.affiliation: Department of Biophysical Chemistry, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Torbjörn
authors.initials: T
authors.lastname: Drakenberg
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Hägerhäll
mesh_terms: D001412:Bacillus subtilis / Q000235:genetics / Q000378:metabolism; D042967:Electron Transport Complex I / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D005816:Genetic Complementation Test; D016297:Mutagenesis, Site-Directed; D009245:NADH Dehydrogenase / Q000235:genetics / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; C000589385:NuoN protein, E coli; D009245:NADH Dehydrogenase; C479743:NuoL protein, E coli; C476247:NuoM protein, E coli; D042967:Electron Transport Complex I