Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape

Abstract

SUMMARY: A fundamental feature of cellular plasma membranes (PM) is asymmetric lipid distribution between the bilayer leaflets. However, neither the detailed, comprehensive compositions of individual PM leaflets, nor how these contribute to structural membrane asymmetries have been defined. We report the distinct lipidomes and biophysical properties of both monolayers in living mammalian PMs. Phospholipid unsaturation is dramatically asymmetric, with the cytoplasmic leaflet being ~2-fold more unsaturated than the exoplasmic. Atomistic simulations and spectroscopy of leaflet-selective fluorescent probes reveal that the outer PM leaflet is more packed and less diffusive than the inner leaflet, with this biophysical asymmetry maintained in the endocytic system. The structural asymmetry of the PM is reflected in asymmetric structures of protein transmembrane domains (TMD). These structural asymmetries are conserved throughout Eukaryota, suggesting fundamental cellular design principles.

The role of prolines and glycine in the transmembrane domain of LAT

“…This is in agreement with previously reported accumulation of voluminous amino acids in the cytosolic part of the TMD of the plasma membrane proteins [9,10]. The authors argue that chemical asymmetry of the plasma membrane, which exhibits higher rigidity in its outer leaflet compared to the inner leaflet, selects proteins with asymmetric TMD in terms of surface area [44]. Voluminous amino acids are randomly distributed over the sequence of LAT TMD.…”

“…Recently, Levental and colleagues reported the role of TMD asymmetry in protein sorting to the plasma membrane [44]. Using bioinformatic approach, they found that proteins at the plasma membrane have smaller surface area in the luminal (exoplasmic) half of their TMD compared to the cytosolic half.…”

Rapid kinetics of lipid second messengers controlled by a cGMP signalling network coordinates apical complex functions inToxoplasmatachyzoites

“…We also saw a large increase in 18:1 in the profile of SM upon TgGC depletion (Figure S3), so that PC and SM where the only lipids to have a large increase in 18:1 ( Figure S3). This may reflect differences in outer and inner leaflet lipid composition of PC and SM (Lorent et al 2020). The FA profile of PE was largely similar between TgGC-depleted and non-depleted cells, however there was an increase in C20:1 and a decrease in C20:4 consistent with the total FA analysis (Figure 2A, C).…”

“…Likely the unsaturated fatty acids incorporate into the outer leaflet and stay there, but the 18:1 is unstable in the outer layer and is transported more rapidly inside the parasite. This is probably because the 18:1 chain is less flexible and disrupts the bilayer so the parasite needs to clear this excess PA to prevent destabilizing the outer leaflet (Figure 4B) (Lorent et al 2020). This shows an unexpected role for oleic acid 18:1 in increasing membrane fluidity as part of normal biophysical processes as part of extracellular motility regulated by TgGC.…”

“…Interestingly, in our results, TgGC-depletion only affected import of PA 18:1 and there was no difference seen in import of PA16:0 (Figure 4E,F). A recent study was able to differentiate the FA chain profiles of the lipids comprising the outer and inner leaflets of the plasma membrane and showed that the outer leaflet had more saturated fatty acids, while the inner membrane lipids had more unsaturated fatty acids (Lorent et al 2020). Likely the unsaturated fatty acids incorporate into the outer leaflet and stay there, but the 18:1 is unstable in the outer layer and is transported more rapidly inside the parasite.…”