ER-TO-GOLGI TRANSPORT IN HIGHER PLANT CELLS: NEW STRUCT & A NEW MECHANISM

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Plant Golgi stacks travel along actin filament bundles in a "Stop and Go" pattern of movement. This Stop and Go pattern is characterized by alternations between rapid straight displacements (go phase), and random wiggling at set locations (stop phase). Wiggling Golgi stacks are associated transiently with ER export sites (tER) marked by the Sec13 protein, a component of the COPII vesicular transport machinery. This suggests that the ER-to-Golgi transport in plant cells takes place during transient interactions between these two compartments. We have investigated Golgi stacks and the ER in Arabidopsis thaliana and in alfalfa root meristem and columella cells, using electron tomography and immunoelectron microscopy. tER sites were identified by the presence of budding COPII vesicles labeled by antisera against AtSar1p. A fraction of the Golgi stacks were located close to ER export sites, and these Golgi appeared to be connected to the tER sites through a ribosome-excluding matrix. ER export sites tend to be organized around the cis-side of close Golgi stacks. All of the COPII vesicles were surrounded by a ribosome-excluding matrix, both while budding and during their transfer to the Golgi. COPII vesicles were seen within the Golgi matrix of docked Golgi stacks and these always displayed a small, irregularly shaped cis-most cisterna. Golgi stacks not connected to ER export sites contained few COPII vesicles in their matrices and lacked characteristic cis-cisternal assembly intermediates. In root columella cells, the ER is confined to cortical region of the cytoplasm. Golgi stacks in the cell cortex were also coupled to ER export sites via a matrix and displayed assembling cis-cisternae; those near the cell's center lacked these structures. Together, these findings demonstrate that COPII vesicle-mediated transport occurs during the "stop" phase of Golgi movement, and that the process of stopping involves the formation of transient links between the ribosome-excluding matrix of forming COPII vesicles and the cis-side of the Golgi matrix.