The travels of mRNAs through all cells large and small

The determination of asymmetry is one of the most important events in cell differentiation and development. Cellular asymmetry in many cases is essential for normal cellular function, as in the polarity of neurons that transmit electrical and chemical signals or the polarity of morphogenetic gradients in oocytes required for patterning in the embryo. The asymmetric localization of specific proteins is a distinct feature of morphological asymmetry. An important mechanism to achieve nonuniform protein distributions is to synthesize specific proteins at or near their sites of function. Observations that RNAs can be localized to distinct regions within cells provided a direct link between gene expression and cellular asymmetry (reviewed in ref 1). The mechanisms involved in mRNA localization have been studied over the last decade and have demonstrated that many localized mRNAs contain noncoding ‘cisacting elements’ or ‘zip codes’ in the 39 UTR that confer localization of the mRNA (2). Many of these noncoding sequences have been shown to be necessary and sufficient for mRNA localization, appear to be unique in each mRNA, and are often present as multipartite elements that vary in length. Proteins that bind the cis-acting elements have been identified which may be involved in distinct aspects of the mRNA localization pathway (2–4). It has been proposed that these proteins act as trans-acting factors and are involved in mRNA transport, anchoring, localized translation, and stability. This review will highlight recent advances in understanding these mechanisms, with an emphasis on somatic cells and conserved features with oocytes.