MRNA encoding the β-subunit of the mitochondrial F1-ATPase complex is a localized mRNA in rat hepatocytes

Abstract

Subcellular mRNA localization has emerged as a mechanism for regulation of gene expression and protein-sorting pathways. Here we describe the different cytoplasmic presentation in rat hepatocytes of two nuclear mRNA species encoding subunits α and β of the mitochondrial F1-ATPase complex. α-F1-ATPase mRNA is dispersed and scattered in the cytoplasm. In contrast, β-F1-ATPase mRNA appears in rounded electron-dense clusters, often in close proximity to mitochondria. Hybridization experiments with β2-microglobulin and β-actin cDNA species reveal an expected subcellular distribution pattern of the mRNA species and a non-clustered appearance. Development does not alter the presentation of β-F1-ATPase mRNA hybrids, although it affects the relative abundance of β-F1-ATPase mRNA clusters in the cytoplasm of the hepatocyte. These findings illustrate in vivo the existence of two different sorting pathways for the nuclear-encoded mRNA species of mitochondrial proteins. High-resolution immunocytochemistry and immunoprecipitation experiments allowed the identification of the β-subunit precursor in the cytoplasm of the hepatocyte, also suggesting a post-translational import pathway for this precursor protein. It is suggested that the localization of β-F1-ATPase mRNA in a subcellular structure of the hepatocyte might have implications for the control of gene expression at post-transcriptional levels during mitochondrial biogenesis in mammals.