The biochemical genetics of mitochondria in mammalian cells

A cell line resistant to antimycin A at a concentration of 15 μM has been isolated following the mutagenesis of the human cell line D98 with N-methyl-N'-nitro-N-nitrosoguanidine. The cell line, designated MA 6.5, has been cloned twice in 15 μM antimycin A, a concentration about ten times that required to kill all parental cells. MA 6.5 is fully sensitive to chloramphenicol and triethyl tin and has an active Krebs cycle. Oxygen consumption of whole cells is unaffected by antimycin at a concentration of 15 μM, although the in vitro activity of succinate-cytochrome c reductase appears to be as sensitive to antimycin as the parental cell enzyme. The resistance phenotype of the mutant is stable in the absence of selection. The proteins synthesised by the mitochondria of MA 6.5 have been examined by sodium dodecyl sulphate electrophoresis and isoelectric focusing. No qualitatively variant protein has been detected, but there are consistent quantitative differences between the profiles of the mutant and parental cell lines on isoelectric focusing. The implications of these findings for the mechanism of resistance are discussed. Preliminary genetic analysis suggests that it is possible to transfer resistance to a closely related cell line employing enucleated cell fragments of MA 6.5 as donors. Some observations on mitochondrially synthesised proteins in a variety of cell lines and hybrids are discussed. These support the view that interspecific differences in the products of mitochondrial protein synthesis on electrophoresis are not solely a result of post-translational modification. Two "reverse segregant" hybrid cell lines, which retain large numbers of human chromosomes, were found to have a predominantly human profile of mitochondrially synthesised proteins on sodium dodecyl sulphate electrophoresis.