Abstract
How membrane proteins distribute and behave on the surface of cells depends on the molecules’ chemical potential. However, measuring this potential, and how it varies with protein-to-protein distance, has been challenging. Here, we present a method we call hydrodynamic trapping that can achieve this. Our method uses the focused liquid flow from a micropipette to locally accumulate molecules protruding above a lipid membrane. The chemical potential, as well as information about the dimensions of the studied molecule, are obtained by relating the degree of accumulation to the strength of the trap. We have used this method to study four representative proteins, with different height-to-width ratios and molecular properties; from globular streptavidin, to the rod-like immune cell proteins CD2, CD4 and CD45. The data we obtain illustrates how protein shape, glycosylation and flexibility influence the behaviour of membrane proteins, as well as underlining the general applicability of the method.
Highlights
Both the interaction and the transfer of information between cells is regulated via membrane proteins acting, among other roles, as receptors, adhesion molecules, or ion transporters
We recently showed that the liquid flow through a micropipette can be used to trap and accumulate molecules in a supported lipid bilayer (SLB), a technique we call hydrodynamic trapping[2,3]
The flow acts on molecules protruding from the lipid bilayer with a drag force whose magnitude can be accurately controlled by varying the flow rate through the pipette and distance to the SLB
Summary
Both the interaction and the transfer of information between cells is regulated via membrane proteins acting, among other roles, as receptors, adhesion molecules, or ion transporters. It is in this way possible to change the local concentration of membrane-bound molecules by orders of magnitude[2]. We selected four membrane-anchored molecules of varying shape, level of glycosylation and flexibility to investigate with our method These molecules were the B vitamin biotin-binding protein streptavidin (SA), and the immune-cell membrane proteins CD2, CD4 and CD45RABC (CD45). The orientation and interaction between these molecules are interesting from a modelling point of view, but will affect how these molecules distribute and behave in the contact between immune cells, which are key factors to better understand immune-cell signalling
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