Understanding mRNA trafficking: are we there yet?

There has been unexpected insight into the part that an mRNA plays in the function of the protein it encodes gleaned from visualizing where an mRNA resides in the cytoplasm. mRNAs can localize to distal domains of the cytoplasm in cells with vast cytoplasmic domains such as the central and peripheral nervous system or oocytes and embryos, but can localize within other somatic cell types as well. Localized translation of such mRNAs in the nervous system can supply distant cellular structures, such as synapses, with required proteins for proper function or development. Similarly, the spatially and temporally localized translation of mRNA early in development spatially segregates developmental cues within the egg to facilitate regional cell fate specification and body plan establishment. All mRNAs do not arrive at these sites of localization, an indication that the mechanisms to get these mRNAs to their destination are specially programmed into the mRNA sequence. Molecular recognition of mRNAs to be localized and the putative connection between the RNA and the cytoskeleton for trafficking to their destination are critical steps in the process. In this review I will present the recent years' progress in understanding these two steps in the mRNA localization process at the molecular level, which is among the most critical determinants for localized translation, since only mRNAs that reach the destination can be locally translated.