Zipcodes and postage stamps: mRNA localisation signals and their trans-acting binding proteins

Messenger RNA (mRNA) localisation is a widespread mechanism within eukaryotic cells that provides local synthesis of proteins close to where they function. In general, this mRNA targeting involves the cytoskeleton and signals within the 3' untranslated region (3'UTR) of the transcript. In this paper, the authors review what is known of the nature of the localisation signals and the proteins that interact with them in animal cells. Specific examples are selected to illustrate the emerging pattern of how these signals are formed by the mRNA and the key RNA-binding proteins. The signals are usually restricted to relatively short regions of the 3'UTR, but their precise nature varies, with both sequence and structure playing key roles. Repeat motifs and functional redundancy also appear as common features of these signals. The trans-acting factors involved in localisation include proteins having other roles in nuclear events, proteins that shuttle between the nucleus and the cytoplasm and translational factors. In addition, there is evidence of homology among these proteins and the mechanisms of localisation across eukaryotic species.