EM imaging of subcellular PTP1B partitioning.

(A) High magnification EM image of several mitochondria expressing mTurquoise-APEX-PTP1B in COS-7 cells. The pronounced DAB precipitate was locally concentrated at the OMM (black arrow) and was notably absent from the intracristal spaces and matrix. Strong staining is also observed along ER subregions in direct apposition to mitochondria and therefore consistent with ER MAM sites (white arrows). (B) Similarly strong DAB precipitation was observed at the OMM of a COS-7 cell expressing mTFP1-APEX-PTP1Btail (black arrow). (C) DAB precipitation was observed at the OMM of a COS-7 cell expressing Tom20-APEX-mTurquoise (black arrow). (D) Image of unlabeled mitochondria from an untransfected COS-7 cell serves as a negative control. The black arrowhead indicates the absence of the labeling of the mitochondrial interior. Scale bars: 500 nm.