INTERACTION OF GLYCOSPHINGOLIPIDS AND GLYCOPROTEINS - THERMOTROPIC PROPERTIES OF MODEL MEMBRANES CONTAINING G(M1) GANGLIOSIDE AND GLYCOPHORIN

High-sensitivity differential scanning calorimetry (DSC) was used to s tudy the mutual interactions between a glycoprotein (human glycophorin , GPA) and a glycosphingolipid (G(M1) ganglioside) embedded in large u nilamellar vesicles composed of dimyristoylphosphatidylcholine (DMPC). The DSC thermograms exhibited by DMPC/G(M1) vesicles, either in the p resence or in the absence of GPA, are resolvable into two components. The relative contribution of the minor component, centered at higher t emperature, to the total enthalpy and its transition temperature incre ase with the concentration of the glycolipid embedded in the vesicles. This minor peak, undetectable in the absence of ganglioside, is indic ative of the occurrence of lateral phase separation and suggests that G(M1) ganglioside-enriched domains are present within the bilayer. At a given concentration of G(M1) embedded in the vesicles, the proportio n of the phase-separated peak is higher in the presence of GPA, sugges ting that the glycoprotein enhances the tendency of G(M1), to segregat e. Experiments investigating the thermotropic behavior of GPA show tha t the temperature of irreversible thermal unfolding of the glycoprotei n inserted in DMPC vesicles, centered at 65.9-degrees-C in the absence of G(M1), is shifted to 57.6-degrees-C when G(M1) is present in the b ilayer. These results indicate that, at least in this experimental sys tem, on the one hand, GPA enhances the tendency of the glycolipid to s egregate within the membrane, and on the other hand, the glycolipid cl usters affect the protein conformation and oligomerization in the memb rane.