Actin Filaments Are Involved in the Maintenance of Golgi Cisternae Morphology and Intra-Golgi pH

Here we examine the contribution of actin dynamics to the architecture and pH of the Golgi complex. To this end, we have used toxins that depolymerize (cytochalasin D, latrunculin B, mycalolide B, and Clostridium botulinum C2 toxin) or stabilize (jasplakinolide) filamentous actin. When various clonal cell lines were examined by epifluorescence microscopy, all of these actin toxins induced compaction of the Golgi complex. However, ultrastructural analysis by transmission electron micros-copy and electron tomography/three-dimensional modelling of the Golgi complex showed that F-actin depolymerization first induces perforation/fragmentation and severe swelling of Golgi cisternae, which leads to a completely disorganized struc-ture. In contrast, F-actin stabilization results only in cisternae perforation/fragmenta-tion. Concomitantly to actin depolymerization-induced cisternae swelling and disorganization, the intra-Golgi pH significantly increased. Similar ultrastructural and Golgi pH alkalinization were observed in cells treated with the vacuolar Hþ-ATPases inhibitors bafilomycin A1 and concanamycin A. Overall, these results sug-gest that actin filaments are implicated in the preservation of the flattened shape o