The Interaction of Gb 3 Glycosphingolipids with l d and l o Phase Lipids in Lipid Monolayers Is a Function of Their Fatty Acids.

The glycosphingolipid Gb 3 is a specific receptor of the bacterial Shiga toxin (STx). Binding of STx to Gb 3 is a prerequisite for its internalization into the host cells, and the ceramide's fatty acid of Gb 3 has been shown to influence STx binding. In in vitro studies on liquid ordered ( l o )/liquid disordered ( l d ) coexisting artificial membranes, Shiga toxin B (STxB) binds solely to l o domains, thus harboring Gb 3 concomitant with an observed lipid redistribution process. These findings raise the question of how the molecular structure of the fatty acid of Gb 3 influences the interaction of Gb 3 with the different lipids preferentially either found in the l o phase, namely, sphingomyelin and cholesterol, or in the l d phase. We addressed this question by using a series of synthetically available and unlabeled Gb 3 glycosphingolipids carrying different long chain C 24 fatty acids (saturated, monounsaturated, and α-hydroxylated). In conjunction with surface tension experiments on Langmuir monolayers, we quantified the excess of free energy of mixing of the different Gb 3 species in monolayers composed of either sphingomyelin or cholesterol or composed of a fluid phase lipid (DOPC). From a calculation of the total free energy of mixing, we conclude that mixing of the saturated Gb 3 species with the l d lipid DOPC is energetically less favorable than all other combinations, while the unsaturated species mix equally well with the l o phase lipids sphingomyelin and cholesterol and the l d phase lipid DOPC. Furthermore, we found that STxB partially penetrates in mixed lipid monolayers (DOPC/sphingomyelin/cholesterol) containing the Gb 3 sphingolipid with a saturated or a monounsaturated C 24 fatty acid. The maximum insertion pressure, as a measure for protein insertion, is >30 mN/m for both Gb 3 molecules and is not significantly different for the two Gb 3 species.