Thyroid hormone regulation of nuclear‐encoded mitochondrial inner membrane polypeptides of the liver

Abstract

The effects of thyroid hormone on nuclear‐encoded mitochondrial inner membrane proteins was investigated by in vitro translation of the endogenous mRNA present in a postmitochondrial fraction from the livers of rats treated in vivo with hormone. The levels of the mRNAs were estimated by quantitative immunoabsorption of the translation mixture. Total protein synthesis was increased 2.6‐fold after 4 days of in vivo hormone treatment, but only 10–15% of the polypeptides were dramatically altered (> 5‐fold). Among the most highly elevated were cytochrome c₁ (> 10‐fold increase) and the Rieske iron‐sulfur protein of the cytochrome bc₁ complex. Other inner membrane proteins (core protein 1, β subunit of F₁ ATPase, subunit IV of cytochrome oxidase, 3‐hydroxybutyrate dehydrogenase) and non‐mitochondrial proteins (rat serum albumin, β₂‐microglobulin) were not altered significantly by hormone treatment. Cytochrome c₁ and the Rieske protein increased after 12 h of hormone treatment, a relatively early response in mammalian mitochondrial biogenesis. The possible significance of this response for the regulation of mitochondrial synthesis and assembly is discussed.