The Charge Properties of Phospholipid Nanodiscs

Abstract

Phospholipids (PLs) are a major, diverse constituent of cell membranes. PL diversity arises from the nature of the fatty acid chains, as well as the headgroup structure. The headgroup charge is thought to contribute to both the strength and specificity of protein-membrane interactions. Because it has been difficult to measure membrane charge, ascertaining the role charge plays in these interactions has been challenging. Presented here are charge measurements on lipid Nanodiscs at 20°C in 100 mM NaCl, 50 mM Tris, at pH 7.4. Values are also reported for measurements made in the presence of Ca(2+) and Mg(2+) as a function of NaCl concentration, pH, and temperature, and in solvents containing other types of cations and anions. Measurements were made for neutral (phosphatidylcholine and phosphatidylethanolamine) and anionic (phosphatidylserine, phosphatidic acid, cardiolipin, and phosphatidylinositol 4,5-bisphosphate (PIP2)) PLs containing palmitoyl-oleoyl and dimyristoyl fatty acid chains. In addition, charge measurements were made on Nanodiscs containing an Escherichia coli lipid extract. The data collected reveal that 1) POPE is anionic and not neutral at pH 7.4; 2) high-anionic-content Nanodiscs exhibit polyelectrolyte behavior; 3) 3 mM Ca(2+) neutralizes a constant fraction of the charge, but not a constant amount of charge, for POPS and POPC Nanodiscs; 4) in contrast to some previous work, POPC only interacts weakly with Ca(2+); 5) divalent cations interact with lipids in a lipid- and ion-specific manner for POPA and PIP2 lipids; and 6) the monovalent anion type has little influence on the lipid charge. These results should help eliminate inconsistencies among data obtained using different techniques, membrane systems, and experimental conditions, and they provide foundational data for developing an accurate view of membranes and membrane-protein interactions.

Figure 1

Free-boundary electrophoresis of 30POPS Nanodiscs. (A) Raw intensity scans show boundary movement from left to right as an intensity increase where the boundary has passed. (B) Distribution of Z_DHH.

Figure 2

Fractional charge of MSP1D1 (black) and MSP1E3D1 (gray) Nanodiscs with varying PS content. A ratio of one means that the raw charge of the molecule is fully expressed in solution, with no neutralization from any counterions. Ratios below one mean that charge neutralization is occurring.

Figure 3

Fractional Z_DHH of MSP1D1 POPC and POPS Nanodiscs in the presence of 3 mM CaCl₂ (Table 2).

Figure 4

Comparison of the fractional Z_DHH between POPS Nanodiscs in the presence of 3 mM Ca²⁺ (black) and POPA Nanodiscs in the presence of Mg²⁺ (gray).

Figure 5

Comparison of the fractional Z_DHH of 30POPS Nanodiscs and 10PIP₂ Nanodiscs in the presence of 3 mM Ca²⁺.