Both Detergent Effects Upon Domain Size and Transmembrane Protein Length Effects Upon Domain Binding Suggest that Hydrophobic Mismatch can Control the Properties of Ordered Membrane Domains (“Rafts”)

Abstract

Hydrophobic mismatch between co-existing domains potentially plays an important role in determining domain size and domain interaction with transmembrane proteins. We recently found that in bilayers with the potential to form co-existing Ld and Lo domains, the detergent Triton X-100 enlarges ordered domain size without perturbing ordered domain formation. This can be explained if the effect of detergent is greatest at the boundary between Lo and Ld domains, which would be the case if the main effect of detergent bound to the membranes is to alter the difference between Lo and Ld bilayer width. In other studies, we found that for a multi-transmembrane segment protein, perfringolysin O (PFO), altering the length of transmembrane sequences controls affinity of the protein for membranes domains in a fashion that is dependent upon domain bilayer width. In bilayers with co-existing Ld and Lo domains, PFO with shortened transmembrane segments preferred to partition into the membrane domains with a thin bilayer width, while PFO with lengthened transmembrane segments preferred to partition into domains forming a wider bilayer. PFO with intermediate length, wild type, transmembrane segments exhibited intermediate behavior. This relative bilayer width preference was the same whether the Lo domains were the thinner or wider membrane domains. The effect of transmembrane length upon domain localization was observed both in vesicles that have domains large enough to see by light microscopy and in vesicles with sub-microscopic domains, in which domain affinity was assayed with FRET. Thus, for both of detergent effects and protein-domain association experiments it is likely that hydrophobic mismatch is a key parameter.