23 June 2005: Waltzing lipids slow under heavy polymers

Membrane-bound lipids slow down - like heavily-laden porters - when large molecules land on top of them, report US chemists.

Liangfang Zhang and Steve Granick from the University of Illinois, Urbana, recorded the mobility of small numbers of lipid molecules in a membrane consisting of two lipid layers (a bilayer). Their findings are important because the mobility of lipids that make up simple membranes have only been investigated by looking at the way that larger numbers of lipid molecules move around. 

Zhang and Granick used fluorescence correlation spectroscopy (FCS), which measures small fluctuations in the light given off by fluorescent molecules. ’The measurement method is somewhat like shining a floodlight at one spot on a dance floor, with couples waltzing in and out of the light,’ explained Granick. 

’We shine a near-infrared laser at a very small spot on the bilayer, and watch the motion of fluorescing molecules waltzing in and out of the illuminated region. By analysing how fast the fluorescence switches on and off, we can measure the rate of mobility.’

The chemists created a flat bilayer made of the phospholipid 1,2-dilauroyl-sn-glycero-3-phosphocholine. They then incorporated the fluorescent compound rhodamine B into the bilayer, at a concentration that meant only one molecule of rhodamine B resided in each small area (around 0.35?m in diameter) being analysed by FCS.

Allowing polymers of different sizes to adsorb onto the surface of the bilayer caused the lipids underneath the polymer to slow down, with the rate of the slowdown dependent on the size of the polymer. The slowdown affected the lipids in both the upper and lower layers of the membrane.

This kind of behaviour hasn’t been seen in membranes before, the researchers report, and has implications for understanding the way proteins interact with cellular membranes. ’For example, lipid movement could affect protein distributions in the membrane and influence docking, formation of synapses, and other membrane-mediated functions,’ said Granick. Jon Evans