The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.