Abstract
Sphingomyelin (SM) is a mammalian lipid mainly distributed in the outer leaflet of the plasma membrane (PM). We show that peripheral myelin protein 2 (PMP2), a member of the fatty-acid-binding protein (FABP) family, can localize at the PM and controls the transbilayer distribution of SM. Genetic screening with genome-wide small hairpin RNA libraries identifies PMP2 as a protein involved in the transbilayer movement of SM. A biochemical assay demonstrates that PMP2 is a phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-binding protein. PMP2 induces the tubulation of model membranes in a PI(4,5)P2-dependent manner, accompanied by the modification of the transbilayer membrane distribution of lipids. In the PM of PMP2-overexpressing cells, inner-leaflet SM is increased whereas outer-leaflet SM is reduced. PMP2 is a causative protein of Charcot-Marie-Tooth disease (CMT). A mutation in PMP2 associated with CMT increases its affinity for PI(4,5)P2, inducing membrane tubulation and the subsequent transbilayer movement of lipids.
Highlights
A characteristic feature of the plasma membrane (PM) in mammalian cells is the transbilayer asymmetry of its lipids
The outer-leaflet SM labeled with EGFP-nontoxic lysenin (NT-Lys) decreased dramatically in the SMase-overexpressing cells (Figure 1A)
The selective labeling of the control cells with EGFP-NT-Lys indicated that no leakage of SMase to the medium from inside the SMase-overexpressing cells occurred
Summary
A characteristic feature of the plasma membrane (PM) in mammalian cells is the transbilayer asymmetry of its lipids. Phosphatidylcholine (PC) is mainly located in the outer leaflet, whereas phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) are almost exclusively located in the inner leaflet (Doktorova et al, 2020; Fujimoto and Parmryd, 2017; Kobayashi and Menon, 2018; van Meer, 2011). Sphingomyelin (SM) is a major sphingolipid, comprising approximately 10% of the total phospholipids in mammalian cells. Several approaches have shown that SM is mainly distributed in the outer leaflet of the PM, creating a barrier to the extracellular environment (Murate et al, 2015; Pomorski and Menon, 2006; van Meer et al, 2008). The molecular mechanisms underlying the maintenance of the SM distribution are not fully understood, and no protein that catalyzes the transbilayer movement (flip-flop) of SM has been identified (Roland and Graham, 2016)
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