Artificial ER-Mitochondrion Tethering Restores Erg6 Localization and Lipid Droplet Formation in Hansenula polymorpha Δpex23 and Δpex29 Cells.

Pex23 proteins are a family of fungal endoplasmic reticulum proteins. Hansenula polymorpha contains four members, two of which, Pex24 and Pex32, function in endoplasmic reticulum-peroxisome membrane contact sites. In the absence of the other two members, Pex23 and Pex29, mitochondria are fragmented and lipid droplet numbers are reduced. We here show that in Δpex23 and Δpex29 cells an increased portion of the lipid droplet protein Erg6 (C24-methyltransferase), an enzyme involved in ergosterol biosynthesis, localizes to mitochondria. Erg6 relocalization and the reduction in lipid droplet numbers are suppressed by an artificial endoplasmic reticulum-mitochondrion tether protein. Sterol measurements showed that the presence of Erg6 at mitochondria did not cause major changes in the overall sterol composition. Our findings suggest that Pex23 and Pex29 play a role in endoplasmic reticulum-mitochondrion contact sites which prevent mitochondrial mislocalization of Erg6.