The mRNA encoding activity-regulated cytoskeleton-associated protein (Arc) is known to be targeted to dendritic regions that have received strong synaptic inputs. However, the cis-acting elements in Arc mRNA that mediate dendritic targeting have not been identified. To identify the dendritic targeting element (DTE) in rat Arc mRNA, we expressed reporter mRNAs containing various regions of Arc in primary hippocampal neurones and analysed their subcellular distribution by in situ hybridization. Here, we report that the 3'-untranslated region of rat Arc mRNA contains a 350-nucleotide DTE with strong dendritic targeting activity and another 370-nucleotide sequence with weaker dendritic targeting activity. The 350-nucleotide DTE does not share any obvious sequence similarity with other known DTEs previously reported.

Model systems, such as plasma membrane‐permeabilized cells, isolated phragmoplasts and membrane ghosts, were prepared from tobacco BY‐2 cells and used for studies of the plant cytoskeleton. The use of membrane‐permeabilized cells enabled us to demonstrate the occurrence in the phragmoplast of the translocation of microtubules away from the equatorial plane and led us to the isolation of a kinesin‐like microtubule‐translocating protein from phragmoplasts. Autoradiographic experiments using preparations of isolated phragmoplasts revealed that 1,3‐β‐glucan was synthesized at the equatorial plane of isolated phragmoplasts and xyloglucan was synthesized in the Golgi apparatus. Membrane ghosts from which pre‐existing cortical microtubules had been removed by a cold solution of CaCl2 bound microtubules that had polymerized in vitro, but those from which pre‐existing microtubules had been removed by KCl did not. We used membrane ghosts that had been pretreated with KCl in an attempt to identify a microtubule‐plasma membrane cross‐linking activity in a fraction of a cell extract. We found that a fraction solubilized by KCl from isolated cortical microtubules had such activity.