Figure 1. Hypothetical scheme of mRNA localization to specific cytoplasmic regions in polarized cells. A current model proposes that mRNAs are marked for transport in the cell nucleus. A nuclear RNA-binding protein, RBP-I, binds to specific cis-acting sequences and accompanies the mRNA into the cytoplasm. There, the complex is joined by additional factors, such as RBP-II and a motor protein that fixes the complex to cytoskeletal elements (either microfilaments or microtubules, depending on the cell type). RBP-II and the motor protein may associate sequentially or, alternatively, as a preformed complex. After delivery of the cargo at the ultimate destination, the ribonucleoprotein complex is reorganized in such a way as to anchor the mRNA by yet another factor, the anchor protein. While in transit, the mRNA is silenced translationally. Protein synthesis is initiated by unknown mechanisms. Recruitment of the translational machinery presumably involves additional protein factors that are not specifically shown in this scheme. (Reprinted with permission from Mohr and Richter 2001)