Cells invest significant energetic resources to maintain a compositional asymmetry between the two apposing leaflets of the plasma membrane (PM), emphasizing the importance of this distribution for cell homeostasis. For example, phospholipid asymmetry produces a negative surface potential on the cytoplasmic PM leaflet, which facilitates binding of positively charged proteins. Despite the central involvement of PM asymmetry in various physiological processes, the distinct, detailed, comprehensive compositions of the PM leaflets are not known, nor is the influence of asymmetric lipid distribution on leaflet biophysical properties. We therefore combined a detailed lipidomic analysis of plasma membrane leaflet compositions with an investigation of leaflet-specific biophysical leaflet properties. In human red blood cell membranes, we describe for the first time the detailed lipidomics of both bilayer leaflets, including a dramatic disparity in lipid acyl chains, with the inner plasma membrane leaflet containing approximately two-fold more acyl chain unsaturations compared to the outer leaflet. Membranes containing highly unsaturated lipids are typically characterized by reduced lipid packing and order compared to those with more saturated acyl chains, suggesting the potential for an order asymmetry in living PMs. Using an environment-sensitive probe (Di4), we indeed observed a large lipid order difference between the two PM leaflets. The outer leaflet is tightly packed, approximately comparable to a liquid ordered “raft” phase. In contrast, the inner leaflet is much more disordered. This phase asymmetry is largely maintained during endocytosis. An analysis of the human proteome revealed that transmembrane proteins have asymmetric shape in their transmembrane domains, but only in organelles which participate in exo- or endocytosis. This asymmetric shape maintains an energetically favourable interaction with the asymmetrically packed membranes, and we conclude that those shapes have co-evolved with the lipid phase asymmetries of living membranes. The function of the observed phase asymmetry remains to be definitively determined, but we speculate that this arrangement is a “best of both worlds”, combining low permeability and high mechanical robustness in the exoplasmic leaflet with high mobility and diffusion in the cytoplasmic leaflet required for dynamic intracellular signaling.

Compositional and biophysical asymmetry in mammalian membrane bilayers

2019 Spring National Meeting

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