When we think of lipids phase separating in a cell membrane we usually think of this process occurring symmetrically, i.e. with like on top of like (this is described as a registered phase). If we consider the simplest case of two lipids, one saturated (A), one unsaturated (B), then if their lengths are sufficiently different (i.e. a high degree of hydrophobic mismatch), then theory suggests that A/B and B/A is energetically more favourable than A/A and B/B. Such an asymmetric arrangement is described as an anti-registered phase. The theoretical paper demonstrated the effect using a very simple model that was a long way from biology.
In collaboration with two condensed matter physicists, John Williamson and Peter Olmsted, we ran a large number of coarse-grained simulations using the MARTINI forcefield using a mixture of 3 lipids that has been previously shown to phase separate. By varying the number of beads in the tail of the saturated lipid, we were able to increase or decrease the period of time the system spent anti-registered. The theory also predicts that decreasing the size of the periodic box will favour anti-registration, which we were also able to confirm.
This is important, because it is usually assumed that lipids in a cell membrane phase separate in a registered manner, leading to local regions enriched in cholesterol that are usually called ‘lipid rafts’. This study, when combined with our work showing that the cytoskeleton can lead to membrane compartmentalisation suggests that there could be small, dynamic patches of anti-registered lipids forming in the corals produced by the crowding of membrane proteins and the effects of the cytoskeleton.
What is really nice about this study is how it came about; I was at the Biophysics meeting in Baltimore in early 2015 tweeting away and bumped into John Williamson. We went for a coffee with Peter Olmsted and they told me how they’d noticed a tiny uptick in the percentage of anti-registration at the start of one of simulations in this paper and that this might agree nicely with their new theory. The saturated lipid in those simulations had 4 beads in each tail, so I agreed to try increasing it to 5 beads per tail and seeing if that led to a prolonged period of anti-registration.
Sure enough, it did, hence the paper.
This is the third and last in a set of three papers that bring my research on cell signalling and membranes in the SBCB group within the Department of Biochemistry to a close and is available to download here for free.