Mouse Apolipoprotein L9 is a phosphatidylethanolamine-binding protein

Mouse Apolipoprotein L9 (ApoL9) is an understudied cytoplasmic, interferon-inducible protein. The details of its intracellular localization and normal cellular functions are unclear. We report here that ApoL9 localizes to small puncta diffusely distributed in the cytoplasm, as well as to larger granules of varying size and number that are similar to aggresome-like induced structures (ALIS) and contain the autophagy receptor Sqstm1/p62, the autophagosome marker Lc3, and ubiquitin. Transfection of B16F10 mouse melanoma cells stably expressing ApoL9 (B16F10^L9) with certain liposome-based transfection reagents causes dramatic disturbances in its subcellular distribution. We reasoned that these disturbances may be due to the interaction of ApoL9 with dioleoylphosphatidylethanolamine (DOPE), the helper lipid component of several transfection reagents. Recombinant ApoL9 produced in E. coli, as well as ApoL9 expressed in HEK293T cells, specifically bind phosphatidylethanolamine (PE) in vitro. ApoL9 is expressed at high levels in liver and brain, organs enriched in PE. Since PE is known to facilitate replication of positive strand RNA viruses, we examined the role of ApoL9 during replication of Japanese encephalitis virus (JEV), a positive strand virus of the family Flaviviridae. JEV titres in B16F10^L9 cells are higher than those in B16F10 cells. We propose that ApoL9 is a PE-binding protein that may have important roles in several cellular processes that involve this phospholipid.