Erythroid differentiation dependent interaction of VPS13A with XK at the plasma membrane of K562 cells

Mutations of the bridge-like lipid transport protein VPS13A and of the lipid scramblase XK result in Chorea Acanthocytosis (ChAc) and McLeod syndrome (MLS) respectively, two similar conditions involving neurodegeneration and deformed erythrocytes (acanthocytes). VPS13A binds XK, suggesting a model in which VPS13A forms a lipid transport bridge between the ER and the plasma membrane (PM) where XK resides. However, studies of VPS13A in HeLa and COS7 cells showed that this protein localizes primarily at contacts of the ER with mitochondria. Overexpression of XK in these cells redistributed VPS13A to the biosynthetic XK pool in the ER but not to PM localized XK. Colocalization of VPS13A with XK at the PM was only observed if overexpressed XK harbored mutations that disengage its VPS13A binding site from an intramolecular interaction. As the acanthocytosis phenotype of ChAc and MLS suggests a role of the two proteins in cells of the erythroid lineage, we explored their localization in K562 cells, which differentiate into erythroblasts upon hemin addition. When tagged VPS13A was overexpressed in hemin treated K562 cells, robust formation of ER-PM contacts positive for VPS13A were observed and their formation was abolished in XK KO cells. ER-PM contacts positive for VPS13A were seldomly observed in undifferentiated K562 cells, in spite of the presence of XK in these cells at concentrations similar to those observed after differentiation. These findings reveal that the interaction of VPS13A with XK at ER-PM contacts requires a permissive state which depends upon cell type and/or functional state of the cell.