Fig. 3.

Protein traffic steps mediated by PACS-1 and PACS-2. (A) PACS-1 mediates the sorting of client proteins from late endosomes (LE) to the TGN, from early endosomes (EE) to the plasma membrane (PM), as well as delivery to the primary cilium. PACS-2 mediates the localization of cargo proteins to the ER, from early endosomes to the TGN or plasma membrane, and also promotes MAM integrity. (B) HIV-1 Nef usurps the sorting steps mediated by PACS-2 and PACS-1 to downregulate the levels of cell surface MHC-I in CD4⁺ T-cells. Nef binds to Akt-phosphorylated PACS-2 on early endosomes (Atkins et al., 2008; Dikeakos et al., 2012, and L.T. and G.T., unpublished results). This allows Nef to traffic to the TGN region where it binds and activates a Src family kinase (SFK; Hck, Src or Lyn). The Nef–SFK complex then recruits ZAP-70 (in T-cells) or Syk (in monocytes and other cell types) and a class I PI3K, which increases the level of PIP₃ (maroon circles) at the plasma membrane. This recruits an ARF6 GEF that accelerates MHC-I internalization through an ARF6-regulated endocytic pathway. Nef diverts the internalized MHC-I molecules from their local recycling compartment (dashed line) and combines with AP-1 and PACS-1 to transport MHC-I through early and late endosomes and sequester it in the TGN. The identity of the precise compartment containing Nef, MHC-I, AP-1 and PACS-1 is under investigation. This MHC-I downregulation pathway protects HIV-1 from CD8⁺ T-cell killing thereby allowing the virus to evade immune surveillance. Thus, small-molecule inhibition of the multi-kinase complex re-exposes MHC-I on the cells surface and sensitizes HIV-1-infected cells to CD8⁺ T-cell killing. The steps shown here depict the ‘signaling mode’ of HIV-1 Nef-induced immune evasion, which HIV-1 implements during the first 48 h post infection. A detailed discussion of Nef-induced immune evasion is presented elsewhere (Dikeakos et al., 2010; Pawlak and Dikeakos 2015).