Trans interactions between galactosylceramide and cerebroside sulfate across apposed bilayers

The two glycosphingolipids galactosylceramide (GalC) and its sulfated form, cerebroside sulfate (CBS), are present at high concentrations in the multilayered myelin sheath and are involved in carbohydrate-carbohydrate interactions between the lipid headgroups, In order to study the structure of the complex of these two glycolipids by Fourier transform infrared (FTIR) spectroscopy, GalC dispersions were combined with CBS dispersions in the presence and absence of Ca2+. The FTIR spectra indicated that a strong interaction occurred between these glycolipids even in the absence of Ca2+. The interaction resulted in dehydration of the sulfate, changes in the intermolecular hydrogen bonding interactions of the sugar and other oxygens, decreased intermolecular hydrogen bonding of the amide C=O of GalC and dehydration of the amide region of one or both of the lipids in the mixture, and disordering of the hydrocarbon chains of both lipids. The spectra also show that Ca2+ interacts with the sulfate of CBS. Although they do not reveal which other groups of CBS and GalC interact with Ca2+ or which groups participate in the interaction between the two lipids, they do show that the sulfate is not directly involved in interaction with GalC, since it can still bind to Ca2+ in the mixture. The interaction between these two lipids could be either a lateral cis interaction in the same bilayer or a trans interaction between apposed bilayers. The type of interaction between the lipids, cis or trans, was investigated using fluorescent and spin-label probes and anti-glycolipid antibodies. The results confirmed a strong interaction between the GalC and the CBS microstructures, They suggested further that this interaction caused the CBS microstructures to be disrupted so that CBS formed a single bilayer around the GalC multilayered microstructures, thus sequestering GalC from the external aqueous phase. Thus the CBS and GalC interacted via a trans interaction across apposed bilayers, which resulted in dehydration of the headgroup and interface region of both lipid bilayers, The strong interaction between these lipids may be involved in stabilization of the myelin sheath.