Maintenance of Golgi structure and function depends on the integrity of ERexport

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitati ve fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We fou nd that all classes of Golgi components are dynamically associated with thi s organelle, contrary to the prediction of the stable organelle model. Enzy mes and recycling components are continuously exiting and reentering the Go lgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without di srupting COPII-dependent ER export machinery (by brefeldin A treatment or e xpression of Arf1 [T31N]), the Golgi structure disassembles, leaving no res idual Golgi membranes. Rather, all Golgi components redistribute into the E R, the cytoplasm, or to ER exit sites still active for recruitment of selec tive membrane-bound and peripherally associated cargos. A similar phenomeno n is induced by the constitutively active Sari [H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a ju xtanuclear region. In cells expressing Sar1[T39N], a constitutively inactiv e form of Sari that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These r esults argue against the hypothesis that the Golgi apparatus contains stabl e components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose me mbranes can be nucleated and are maintained by the activities of the Sar1-C OPII and Arf1-coatomer systems.