The Lipids that Get the Invite to a Caveolar Neck Party

Abstract

Caveolae are plasma membrane (PM) invaginations that appear as bulbs protruding into the cytoplasm. individual bulbs are connected to the bulk membrane by a highly curved neck with an internal pore. Like other highly curved biological structures (e.g., pores created by membrane compartment fusion and clathrin coated endocytic sites before the vesicle pinches off), caveolar necks are created by cellular protein machinery, and the work done by proteins to create and sustain the high curvature depends on the lipid membrane material properties and distributions. Understanding and quantifying lipid equilibrium distributions requires atomistic detail to resolve specific interactions, but also long timescales to allow sufficient lipid diffusion. To meet both of these demands, we used the special-use Anton supercomputer to attain continuous, long timescale trajectories of lipid diffusion on two pores with different internal radii and a planar bilayer as a control. We used a PM mimetic that balances the necessary lipidic complexity with the necessary simplicity to quantify important lipid-lipid interaction motifs and redistributions on a timescale accessible to all-atom simulations. The multi-microsecond simulations allowed us to test two core hypotheses: i) that ganglioside GM3 and palmitoyl sphingomyelin redistribute to areas of positive curvature, and ii) that cholesterol will preferentially flip-flop into the outer leaflet. Because of the diverse curvature accessible on a caveolar pore, the simulations also allow insight into whether lipids redistribute with respect to both net curvature and Gaussian curvature. Preliminary results indicate that cholesterol is depleted in the narrower neck pore and that phosphatidylcholine lipids are enriched in pore neck pores.