ASH1 mRNA localization in three acts.

Novel green fluorescent protein (GFP) labeling techniques targeting specific mRNA transcripts reveal discrete phases of mRNA localization in yeast: packaging, transport, and docking. In budding yeast, ASH1 mRNA is translocated via actin and myosin to the tip of growing cells. A GFP-decorated reporter transcript containing the ASH1 3' untranslated region gRNA(ASH1) forms spots of fluorescence localized to a cortical domain at the bud tip, relocates to the mother-bud neck before cell separation, and finally migrates to the incipient bud site before the next budding cycle. The correct positioning of the mRNA requires at least six proteins: She1p-5p and Bud6p/Aip3p. gRNA(ASH1) localization in mutant strains identified three functional categories for the She proteins: mRNA particle formation (She2p and She4p), mRNA transport into the bud (She1p/Myo4p and She3p), and mRNA tethering at the bud tip (She5p/Bni1p and Bud6p/Aip3p). Because localization of the mRNA within the bud does not a priori restrict the translated protein, we examine the distribution of a mother-specific protein (Yta6p) translated from a mRNA directed into the bud. Yta6p remains associated with the mother cortex despite localization of the mRNA to the bud. This video essay traces the life history of a localized mRNA transcript, describes the roles of proteins required to polarize and anchor the mRNA, and demonstrates at least one instance where mRNA localization does not effect protein localization.