Molecular characterisation of the 76 kDa iron-sulphur protein subunit of potato mitochondrial complex I

Genes encoding subunits of complex I (EC 1.6.5.3) of the mitochondrial respiratory chain vary in their locations between the mitochondrial and nuclear genomes in different organisms, whereas genes for a homologous multisubunit complex in chloroplasts have to date only been found on the plastid genome. In potato (Solanum tuberosum L.), the gene coding for the mitochondrial 76 kDa iron-sulphur protein is identified in the nuclear genome. The gene is transcribed into polyadenylated mRNA which is most abundant in flowers, and more frequent in tubers than in leaves. The amino acid sequence is well conserved relative to the nuclear-encoded 75 kDa and 78 kDa subunits of Bos taurus and Neurospora crassa, respectively, and to the Paracoccus denitrificans homologue, most prominently in the region presumed to carry the iron-sulphur clusters. Polyclonal antibodies directed against the 78 kDa complex I subunit of N. crassa recognise the 76 kDa polypeptide in potato mitochondrial complex I, and additionally a polypeptide of 75 kDa in solubilised stroma thylakoids from spinach chloroplasts. The 32 amino acid residues long presequence of the potato mitochondrial 76 kDa complex I subunit targets the precursor polypeptide into isolated potato mitochondria but not into isolated chloroplasts. These results suggest that chloroplast stroma thylakoids contain a protein similar in size and antigenicity to, but genetically distinct from, the mitochondrial subunit.