Golgi dispersal during microtubule disruption: regeneration of Golgi stacks at peripheral endoplasmic reticulum exit sites.

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Abstract
Microtubule disruption has dramatic effects on the normal centrosomal localization of the Golgi complex, with Golgi elements remaining as competent functional units but undergoing a reversible "fragmentation" and dispersal throughout the cytoplasm. In this study we have analyzed this process using digital fluorescence image processing microscopy combined with biochemical and ultrastructural approaches. After microtubule depolymerization, Golgi membrane components were found to redistribute to a distinct number of peripheral sites that were not randomly distributed, but corresponded to sites of protein exit from the ER. Whereas Golgi enzymes redistributed gradually over several hours to these peripheral sites, ERGIC-53 (a protein which constitutively cycles between the ER and Golgi) redistributed rapidly (within 15 minutes) to these sites after first moving through the ER. Prior to this redistribution, Golgi enzyme processing of proteins exported from the ER was inhibited and only returned to normal levels after Golgi enzymes redistributed to peripheral ER exit sites where Golgi stacks were regenerated. Experiments examining the effects of microtubule disruption on the membrane pathways connecting the ER and Golgi suggested their potential role in the dispersal process. Whereas clustering of peripheral pre-Golgi elements into the centrosomal region failed to occur after microtubule disruption, Golgi-to-ER membrane recycling was only slightly inhibited. Moreover, conditions that impeded Golgi-to-ER recycling completely blocked Golgi fragmentation. Based on these findings we propose that a slow but constitutive flux of Golgi resident proteins through the same ER/Golgi cycling pathways as ERGIC-53 underlies Golgi Dispersal upon microtubule depolymerization. Both ERGIC-53 and Golgi proteins would accumulate at peripheral ER exit sites due to failure of membranes at these sites to cluster into the centrosomal region. Regeneration of Golgi stacks at these peripheral sites would re-establish secretory flow from the ER into the Golgi complex and result in Golgi dispersal.