HIV-1 Protein Nef Inhibits Activity of ATP Binding Cassette Transporter A1 by Targeting Endoplasmic Reticulum Chaperone Calnexin [Lipids]

August 28th, 2014 by Jennelle, L., Hunegnaw, R., Dubrovsky, L., Pushkarsky, T., Fitzgerald, M. L., Sviridov, D., Popratiloff, A., Brichacek, B., Bukrinsky, M.

HIV-infected patients are at increased risk of developing atherosclerosis, in part due to an altered HDL profile exacerbated by downmodulation and impairment of ATP-Binding Cassette Transporter A1 (ABCA1) activity by the HIV-1 protein Nef. However, the mechanisms of this Nef effect remained unknown. Here, we show that Nef interacts with an endoplasmic reticulum (ER) chaperone calnexin, which regulates folding and maturation of glycosylated proteins. Nef disrupted interaction between calnexin and ABCA1, but increased affinity and enhanced interaction of calnexin with HIV-1 gp160. Nef mutant that did not bind to calnexin did not affect the calnexin-ABCA1 interaction. Interaction with calnexin was essential for functionality of ABCA1, as knock-down of calnexin blocked ABCA1 exit from the ER, reduced ABCA1 abundance, and inhibited cholesterol efflux; the same effects were observed after Nef overexpression. However, the effects of calnexin knockdown and Nef on cholesterol efflux were not additive; in fact, the combined effect of these two factors together did not differ significantly from the effect of calnexin knockdown alone. Interestingly, gp160 and ABCA1 interacted with calnexin differently: while gp160 binding to calnexin was dependent on glycosylation, glycosylation was of little importance for interaction between ABCA1 and calnexin. Thus, Nef regulates activity of calnexin to stimulate its interaction with gp160 at the expense of ABCA1. This study identifies a mechanism for Nef-dependent inactivation of ABCA1 and dysregulation of cholesterol metabolism.