The molecular mechanism by which ATP-binding cassette transporter A1 (ABCA1) mediates cellular binding of apolipoprotein A-I (apoA1) and nascent high-density lipoprotein (HDL) assembly is not well understood.Objective:
To determine the cell surface lipid that mediates apoA1 binding to ABCA1-expressing cells and the role it plays in nascent HDL assembly.Methods and Results:
Using multiple biochemical and biophysical methods, we found that apoA1 binds specifically to phosphatidylinositol (4,5) bis-phosphate (PIP2). Flow cytometry and PIP2 reporter-binding assays demonstrated that ABCA1 led to PIP2 redistribution from the inner to the outer leaflet of the plasma membrane. Enzymatic cleavage of cell surface PIP2 or decreased cellular PIP2 by knockdown of phosphatidylinositol-5-phosphate 4-kinase impaired apoA1 binding and cholesterol efflux to apoA1. PIP2 also increased the spontaneous solubilization of phospholipid liposomes by apoA1. Using site-directed mutagenesis, we found that ABCA1’s PIP2 and phosphatidylserine translocase activities are independent from each other. Furthermore, we discovered that PIP2 is effluxed from cells to apoA1, where it is associated with HDL in plasma, and that PIP2 on HDL is taken up by target cells in a scavenger receptor-BI–dependent manner. Mouse plasma PIP2 levels are apoA1 gene dosage–dependent and are >1 μM in apoA1 transgenic mice.Conclusions:
ABCA1 has PIP2 floppase activity, which increases cell surface PIP2 levels that mediate apoA1 binding and lipid efflux during nascent HDL assembly. We found that PIP2 itself is effluxed to apoA1 and it circulates on plasma HDL, where it can be taken up via the HDL receptor scavenger receptor-BI.