Lipophilic proteins encoded by mitochondrial and nuclear genes in Neurospora crassa

Abstract

Mitochondrial proteins soluble in neutral chloroform-methanol (2:1) were separated from lipids by ether precipitation and resolved by Sephadex G-200 filtration in the presence of dodecylsulfate into two major fractions eluting in the excluded region (peak I) and in a region of an apparent molecular weight 8000 (peak II). Residual phospholipids are found only in peak II. Peak I consists of several aggregated small polypeptides of molecular weights around 8000, which can be disaggregated by mild oxidation with performic acid. Cycloheximide stimulates almost two-fold incorporation of radioactive phenylalanine into peak I proteins but inhibits labelling of peak II proteins by 95%. Chloramphenicol and ethidium bromide inhibit the synthesis of peak I proteins by 70% and 95% respectively, but do not affect labelling of peak II proteins. At least 30% of the translation products of mitochondrial DNA in vitro behave like peak I proteins: they are soluble in organic solvents, they aggregate in dodecylsulfate buffer after removal of phospholipids and they contain species of molecular weights around 8000 that disaggregate upon oxidation. The data strongly suggest that the proteins of peak I are encoded by mitochondrial genes and synthesized on mitochondrial ribosomes, whereas the proteins of peak II are encoded by nuclear genes and synthesized on cytoplasmic ribosomes. Both groups of lipophilic proteins are very similar in their molecular weights, but the mitochondrially coded peak I proteins have the unique property of forming large heat-stable aggregates in the presence of dodecylsulfate.