Ribonucleoprotein complexes in gene expression: Remodeling events and common components in nuclear and mitochondrialmRNA maturation

Messenger RNAs associate co-transcriptionally with several proteins to form ribonucleoprotein (RNP) complexes. Several lines of evidence indicate that these RNP complexes undergo substantial changes in protein composition as the RNAs are processed in the nucleus and exported to the cytoplasm. The work presented in this dissertation describes the isolation, from HeLa cells, of three distinct types of pre-mRNP and mRNP complexes associated with hnRNP A1, a shuttling hnRNP protein thought to bind RNAs from their transcription through their processing and export. Based on their RNA and protein composition, these complexes are likely to represent RNPs at distinct stages in the nucleocytoplasmic shuttling pathway of hnRNP A1 with its bound RNAs. Interestingly, one of these complexes exhibits characteristics of RNPs at late stages of nuclear maturation and likely arises from remodeling of the pre-mRNA containing hnRNP complexes. This remodeling includes removal of non-shuttling hnRNP proteins and recruitment of alternatively spliced isoforms of hnRNP proteins and of LRP130, a recently described PPR motif-containing protein.; Further characterization of LRP 130 revealed that it is a novel RNA-binding protein, whose RNA-binding activity in vitro maps to a domain in its C-terminal end that contains 2 of the 11 predicted PPR motifs. Furthermore, while only a fraction of LRP130 is in nuclei and is bound to at least some of the same RNAs as hnRNP A1, the majority of LRP130 is located within mitochondria where it is directly bound in vivo to mitochondrial polyadenylated RNAs. Targeting to each compartment is likely mediated by specific signals within the N-terminal region of LRP130 and the available evidence suggests that a retrograde transport from mitochondria to nuclei underlies this dual distribution. Within mitochondria, LRP130 is associated with RNP complexes, which are distinct in protein composition from the nuclear RNPs. These LRP130-associated mitochondrial RNPs contain, among others, the α subunit of the mitochondrial Trifunctional Protein as a specific component. Altogether, this work has revealed that specific changes occur in the protein composition of nuclear RNPs during maturation and traffic in vivo and that LRP 130 is dually localized in nuclei and mitochondria and likely participates in RNA metabolism in both compartments.