Fig. 5.

Possible models for the regulation of Bid translocation to mitochondria by PACS proteins and vMIA. (A) Example of an experiment showing that vMIA prevents translocation of Bid–GFP to mitochondria (L.T., T.S., J.A. and G.T., unpublished results). MCF-7 cells expressing Bid–GFP were left untreated (control, top) or transfected with a vector expressing vMIA (blue) followed by treatment with anti-Fas antibody (1 µg/ml) plus cycloheximide (CHX, 20 µg/ml) for 3 h (bottom) to induce Bid translocation. Mitochondria were then labeled with Mitotracker Red. Image analysis showed that anti-Fas antibody concentrated Bid–GFP staining on mitochondria in untransfected cells (yellow asterisk) but not in cells expressing vMIA (white asterisks). Scale bar: 20 µm. (B) Conventional model of Bid regulation. Dephosphorylation of full-length Bid exposes a cleavage site for caspase-8. Caspase-8-mediated cleavage generates tBid, which is then myristoylated and traffics to mitochondria to promote MOMP. (C) Alternative model of Bid regulation. Dephosphorylation of full-length Bid exposes a binding site for PACS-2 or PACS-1. Binding to PACS-2 promotes Bid translocation to mitochondria (solid lines), whereas binding of Bid to PACS-1 interrupts its translocation to mitochondria (upper dashed lines). In HCMV-infected cells, vMIA sequesters PACS-2 to mitochondria (lower dashed lines), thereby preventing Bid recruitment and, ultimately, MOMP. TRAIL-R, TRAIL receptor.