Two trans-acting rat-brain proteins, MARTA1 and MARTA2, interact specifically with the dendritic targeting element in MAP2 mRNAs

Messenger RNA on the move: implications for cell polarity

RNA sorting is an evolutionarily conserved mechanism for establishing asymmetries within a given cell concerning the macromolecular equipment of defined domains. mRNAs as well as non-coding transcripts are delivered to specific subcellular compartments in diverse organisms including developmental systems of Drosophila, Xenopus, ascidiens, zebrafish and echinoderms and in differentiated cells from yeast to mammals. The composition of the RNA localization machinery is complex. Both sequence- or structural motifs within RNA molecules to be transported (cis-acting elements) and various proteins (trans-acting factors) contribute to the localization procedure. Most often these macromolecular ribonucleoprotein complexes exhibit a granular appearance, and granule localization depends on intact microfilaments or microtubules. When delivered to their ultimate destinations mRNAs are anchored to await translational activation at the appropriate timepoint. Beyond doubt, RNA localization plays a pivotal role in embryonic development, where mRNA mislocations cause severe body pattern defects. In terminally differentiated vertebrate cells RNA transport and local on-site translation presumably have an impact on various cellular functions such as cell motility, myelinization of nerve cell axons and nerve cell communications in the central and peripheral nervous system.

Vasopressin mRNA localization in nerve cells: characterization of cis-acting elements and trans-acting factors

mRNA localization is a complex pathway. Besides mRNA sorting per se, this process includes aspects of regulated translation. It requires protein factors that interact with defined sequences (or sequence motifs) of the transcript, and the protein/RNA complexes are finally guided along the cytoskeleton to their ultimate destinations. The mRNA encoding the vasopressin (VP) precursor protein is localized to the nerve cell processes in vivo and in primary cultured nerve cells. Sorting of VP transcripts to dendrites is mediated by the last 395 nucleotides of the mRNA, the dendritic localizer sequence, and it depends on intact microtubules. In vitro interaction studies with cytosolic extracts demonstrated specific binding of a protein, enriched in nerve cell tissues, to the radiolabeled dendritic localizer sequence probe. Biochemical purification revealed that this protein is the multifunctional poly(A)-binding protein (PABP). It is well known for its ability to bind with high affinity to poly(A) tails of mRNAs, prerequisite for mRNA stabilization and stimulation of translational initiation, respectively. With lower affinities, PABP can also associate with non-poly(A) sequences. The physiological consequences of these PABP/RNA interactions are far from clear but may include functions such as translational silencing. Presumably, the translational state of mRNAs subject to dendritic sorting is influenced by external stimuli. PABP thus could be a component required to regulate local synthesis of the VP precursor and possibly of other proteins.

Identification of a cis-acting dendritic targeting element in the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II

In mammalian neurons a selected group of mRNAs, including the transcript encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II, is found in dendrites. The molecular mechanisms underlying extrasomatic RNA trafficking are not well described. It is thought that dendritic transcripts contain cis-acting elements that direct their selective subcellular sorting. Here we report the identification of an extrasomatic targeting element in the 3' untranslated region of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II. In primary hippocampal neurons, this 1200-nucleotide-spanning, cis-acting element is sufficient to mediate dendritic localization of chimeric reporter transcripts. The trafficking signal does not share any striking sequence similarity with a previously characterized dendritic targeting element in transcripts encoding the microtubule-associated protein 2. In dendrites of transfected primary neurons, recombinant RNAs form granules with an average diameter of 0.45 microm that may represent preferential RNA docking sites or multimolecular transport units. These findings imply that extrasomatic sorting of individual dendritic mRNAs involves at least partially distinct molecular mechanisms, as well as large trafficking complexes.

VP-RBP, a protein enriched in brain tissue, specifically interacts with the dendritic localizer sequence of rat vasopressin mRNA

The concept of mRNA localization suggests that this process is mediated by sequences residing in the transcript to which proteins specifically bind and ultimately deliver the mRNA along cytoskeletal elements to specific intracellular destinations. The mRNA encoding the vasopressin (VP) precursor protein is localized to the nerve cell processes both in hypothalamic magnocellular neurons and in primary cultured neurons derived from embryonic rat superior cervical ganglia microinjected with a corresponding eukaryotic expression vector. The last 395 nucleotides of the VP mRNA encompassing part of the coding region, as well as the complete 3'-untranslated region, are sufficient to confer dendritic targeting to a normally nonlocalized reporter transcript. Here we report that, by employing in vitro crosslinking analyses with rat brain proteins and radiolabelled VP transcripts, an RNA-binding protein specifically interacts with the dendritic localizer sequence of the VP mRNA. This protein is enriched in nerve cell tissues. Peripheral tissues and various cell lines contain only low amounts of the binding activity. It therefore represents a candidate protein that may be involved in any aspect related to subcellular VP mRNA sorting in nerve cells, including transport and anchoring of the mRNA and possibly its translational control.