Phospholipid asymmetry in the isolated sarcoplasmic reticulum membrane

Abstract

The total phospholipid content and distribution of phospholipid species between the outer and inner monolayers of the isolated sarcoplasmic reticulum membrane was measured by phospholipase A 2 activities and neutron diffraction. Phospholipase measurements showed that specific phospholipid species were asymmetric in their distribution between the outer and inner monolayers of the sarcoplasmic reticulum lipid bilayer; phosphatidylcholine (PC) was distributed 48 52 ± 2% between the outer and inner monolayer of the sarcoplasmic reticulum bilayer, 69% of the phosphatidylethanolamine (PE) resided mainly in the outer monolayer of the bilayer, 85% of the phosphatidylserine (PS) and 88% of the phosphatidylinositol (PI) were localized predominantly in the inner monolayer. The total phospholipid distribution determined by these measurements was 48 52 ± 2% for the outer/inner monolayer of the sarcoplasmic reticulum lipid bilayer. Sarcoplasmic reticulum phospholipids were biosynthetically deuterated and exchanged into isolated vesicles with both a specific lecithin and a general exchange protein. Neutron diffraction measurements directly provided lipid distribution profiles for both PC and the total lipid content in the intact sarcoplasmic reticulum membrane. The outer/inner monolayer distribution for PC was 47 53 ± 1% , in agreement with phospholipase measurements, while that for the total lipid was 46 54 ± 1% , similar to the phospholipase measurements. These neutron diffraction results regarding the sarcoplasmic reticulum membrane bilayer were used in model calculations for decomposing the electron-density profile structure (10 Å resolution) of isolated sarcoplasmic reticulum previously determined by X-ray diffraction into structures for the separate membrane components. These structure studies showed that the protein profile structure within the membrane lipid bilayer was asymmetric, complementary to the asymmetric lipid structure. Thus, the total phospholipid asymmetry obtained by two independent methods was small but consistent with a complementary asymmetric protein structure, and may be related to the highly vectorial functional properties of the calcium pump ATPase protein in the sarcoplasmic reticulum membrane.