A network of cytosolic (co)chaperones promotes the biogenesis of mitochondrial signal-anchored outer membrane proteins

Signal-anchored (SA) proteins are anchored into the mitochondrial outer membrane (OM) via a single transmembrane segment at their N-terminus while the bulk of the proteins is facing the cytosol. These proteins are encoded by nuclear DNA, translated on cytosolic ribosomes, and are then targeted to the organelle and inserted into its OM by import factors. Recently, research on the insertion mechanisms of these proteins into the mitochondrial OM have gained a lot of attention. In contrast, the early cytosolic steps of their biogenesis are unresolved. Using various proteins from this category and a broad set of in vivo, in organello, and in vitro assays, we reconstituted the early steps of their biogenesis. We identified a subset of molecular (co)chaperones that interact with newly synthesized SA proteins, namely, Hsp70 and Hsp90 chaperones and co-chaperones from the Hsp40 family like Ydj1 and Sis1. These interactions were mediated by the hydrophobic transmembrane segments of the SA proteins. We further demonstrate that interfering with these interactions inhibits the biogenesis of SA proteins to a various extent. Finally, we could demonstrate direct interaction of peptides corresponding to the transmembrane segments of SA proteins with the (co)chaperones and reconstitute in vitro the transfer of such peptides from the Hsp70 chaperone to the mitochondrial Tom70 receptor. Collectively, this study unravels an array of cytosolic chaperones and mitochondrial import factors that facilitates the targeting and membrane integration of mitochondrial SA proteins

Hepatitis B virus targets lipid transport pathways to infect hepatocytes

International audienceBackground and aims: A single hepatitis B virus (HBV) particle is sufficient to establish chronic infection of the liver after intravenous injection, suggesting that the virus targets hepatocytes via a highly efficient transport pathway. We therefore investigated whether HBV utilizes a physiological liver-directed pathway that supports specific host-cell targeting in vivo.Methods: We established an ex vivo system of perfused human liver tissue that recapitulates the liver physiology to investigate HBV liver targeting. This model allowed us to investigate virus-host cell interactions in a cellular microenvironment mimicking the in vivo situation. Results: HBV was rapidly sequestered by liver macrophages within one hour after a virus pulse, but was detected in hepatocytes only after 16 hours. We found that HBV associates with lipoproteins. Electron- and immunofluorescence microscopy corroborated a co-localization in recycling endosomes within peripheral and liver macrophages. Importantly, recycling endosomes accumulated HBV and cholesterol, followed by transport of HBV back to the cell surface along the cholesterol efflux pathway. To reach hepatocytes as final target cells, HBV was able to utilise the hepatocyte-directed cholesterol transport machinery of macrophages.Conclusions: Our results propose that HBV by binding to liver targeted lipoproteins and utilizing the reverse cholesterol transport pathway of macrophages hijacks the physiological lipid transport pathways to the liver to most efficiently reach its target organ. This may involve trans-infection of liver macrophages and result in deposition of HBV in the perisinusoidal space from where HBV can bind its receptor on hepatocyte