Yip1A-knockdown prevents maturation of B. abortus into ER-derived replicative BCVs, and confines BCVs within Lamp2-positive compartments.

(A) Representative electron micrographs of Brucella-infected control (left-hand panel) and Yip1A-knockdown (right-hand panel) cells at 24 hr p.i. Insets are magnifications of the boxed areas on the main image, showing the typical forms of BCVs. In control cells, BCVs can be seen in the form of ER-derived membrane-bound compartments (inset in left-hand panel, defined as ‘I’). Note the presence of ribosomes on the membrane (arrowheads). In Yip1A-knockdown cells, the bacteria were not sequestered into such compartments (inset in right-hand panel, defined as ‘II’). Scale bars are 2 μm. (B) Representative electron micrograph of Brucella-infected IRE1-knockdown cells at 24 hr p.i. Inset is a magnification of the boxed area in the main image, showing that the bacteria were not sequestered into ER-derived membrane. Scale bar is 2 μm. (C) The percentages of the two forms of BCVs (I and II) present in control and Yip1A-knockdown cells at 24 hr p.i. The total numbers of BCVs analyzed were 67 for control cells and 37 for Yip1A-knockdown cells. (D, E) Representative confocal micrographs of control (upper panels), Yip1A-knockdown (middle panels), and IRE1-knockdown (lower panels) cells double-stained for Lamp2 (a marker for late endosomes/lysosomes; green) and B. abortus (red) at 24 hr p.i. BCVs co-localized with Lamp2 are indicated by arrowheads. The infected cells are outlined with white dashed lines. Scale bars are 10 μm. The percentage of Lamp2-positive BCVs was determined, and is shown in the line graph (E). (F) Magnification of the electron micrograph of the Brucella-infected control cell in (A), showing large vacuoles studded locally with ribosomes (arrows). (G-I) Representative confocal micrographs of Brucella-infected control cells at 24 hr p.i. triple-stained for HA-Sec61α (a marker for rough ER; green), B. abortus (red), and Lamp2 (a marker for endosomes/lysosomes; blue) (G). An expression construct for HA-Sec61α was co-transfected with scramble siRNA. The infected cell is outlined with white dashed lines. Scale bars are 10 μm. Magnifications of the boxed areas ‘a’ and ‘b’ are shown in (H) and (I), respectively. Large vacuoles adjacent to replicating bacteria were stained for both Sec61α and Lamp2 (H). B. abortus was co-stained with Sec61α but not with Lamp2 (I).