 SdeA WT, but not its mART mutant (SdeA EE/AA), leads to fragmentation of FAM134B-labeled ER networks ([Figure S7](#mmc1){ref-type="supplementary-material"}A). Similarly, in cells infected with WT *Legionella*, the FAM134B-GFP-labeled ER was more fragmented than in cells infected with *ΔsidE Legionella*. Furthermore, in non-infected cells, the ER forms a dense meshwork with highly branched structures, while cells infected with *Legionella* showed a fragmented ER with larger spaces between ER tubules. Interestingly, cells infected with *ΔsidE Legionella* strain showed an intermediate phenotype between WT and non-infected cells ([Videos S5](#mmc7){ref-type="supplementary-material"} and [S6](#mmc8){ref-type="supplementary-material"}; [Figure S7](#mmc1){ref-type="supplementary-material"}B). Next, to check the effect of SidE on ER sheets and tubules, we coimmunostained the ER tubule (Reep5) and the ER sheet (CLIMP63) markers in infected A549 cells where Reep5 staining showed a highly branched tubular network and CLIMP63 marks the central sheet-like ER. In contrast, upon infection with WT *Legionella*, both the tubular ER and the ER sheets appeared fragmented compared to non-infected cells or *ΔsidE*-strain-infected cells ([Figure 6](#fig6){ref-type="fig"}A). Also, the CLIMP63 staining was more dispersed, and separation between the ER tubules and sheets were less clear ([Figures 6](#fig6){ref-type="fig"}B and 6C).Figure 6Roles of PR Ubiquitination in ER Remodeling and Fragmentation(A) A549 cells were infected with WT and *ΔsidE Legionella* strains. 2 h post-infection cells were fixed and coimmunostained using Reep5 and CLIMP63 antibodies to mark ER tubules and ER sheets, respectively. DAPI marks nucleus and cytosolic bacteria.(B) Images in (A) were used to quantitate area occupied by ER sheets and ER tubules. Ratios of these values are plotted for non-infected, WT, and *ΔsidE* strains. Data represent 40 cells taken from three independent experiments.(C) Cells in (A) with area ratio \>0.6 are classified as cel