Messenger RNA localization occurs in the cytoplasm and allows temporal and spatial regulation of gene expression. In yeast, the localization of ASH1 mRNA to the tip of budding cells allows the asymmetric sorting of Ash1 protein, which has a key function in the regulation of mating-type switching. After cell division, asymmetric distribution of Ash1p restricts mating type switching to only the mother cell. The cytoplasmic transport of ASH1 mRNA to the bud tip depends on the myosin Myo4p, its adaptor She3p, and the specific RNA binding protein She2p. Three additional trans-acting factors Khd1p, Puf6p and Loc1p are involved in this process. All known RNA-binding proteins of ASH1 mRNA have revealed a nuclear connection, when following their cellular distribution by indirect immunofluorescence. Thus, an early step in the localization pathway might be the early recruitment of specific trans-acting factors to the mRNA already in the nucleus. The aim of this thesis was to investigate how nuclear key events such as early binding to localized transcripts and the subsequent assembly into a nuclear RNP can account for effective RNA localization. Following the route of She2p, it was possible to show nucleo-cytoplasmic shuttling of this RNA binding protein. Moreover, ASH1 mRNA and She2p were found accumulated within the nucleolus upon arrest of mRNA export. Interestingly, two additional trans-acting factors, Loc1 and Puf6p, both involved in ASH1 mRNA localization are also nucleolar proteins. Moreover, She2’s nuclear history seems to be important for an effective sorting of Ash1p. When restricting ASH1-She2p association to the cytoplasmic compartment artificially, the ASH1 mRNA was still localized but was prematurely translated during its transport. This suggests that nuclear RNP assembly has an influence on the later stages of cytoplasmic translational control. The nucleolus might represent the appropriate cellular compartment to provide the spatial framework for the assembly of localization competent RNPs since many RNPs are assembled in this region.