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 alpha and beta of the mitochondrial F1-ATPase complex. alpha-F1-ATPase mRNA is dispersed and scattered in the cytoplasm. In contrast, beta-F1-ATPase mRNA appears in rounded electron-dense clusters, often in close proximity to mitochondria. Hybridization experiments with beta2-microglobulin and beta-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 beta-F1-ATPase mRNA hybrids, although it affects the relative abundance of beta-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 beta-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 beta-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.