Optical and scanning probe analysis of glycolipid reorganization upon concanavalin A binding to mannose-coated lipid bilayers

Molecular level associations between components within the lipid bilayer as proteins complex to the membrane surface are fundamental to the complete understanding of protein-membrane interactions. To characterize these protein-induced membrane-organizational processes, we have prepared a new fluorescently labeled glycolipid (PSMU) that enables monitoring of glycolipid aggregation within the lipid membrane. The glycolipid's mannosamine headgroup was specifically recognized by the lectin concanavalin A (Con A). Fluorescence studies with liposomes composed of 5 mol % PSMU/distearylphosphatidylcholine found that the membrane reorganized in response to Con A adsorption. Initially aggregated structures of glycolipid were dispersed as a consequence of specific affinity with the lectin through steric restrictions imposed by other bound Con A, the distance between mannosyl receptor sites, and possible protein insertion events. The protein binding and membrane reorganizational process was slow (ca. days). An association constant for Con A with the glycolipid membrane was estimated to be 3 x 10(6) M-1, around 2 orders of magnitude higher than that for methyl-D-alpha-mannopyranoside. Nanoscale imaging with the atomic force microscope found that the glycolipid formed 10 nm wide dendrite structures throughout the membrane and that the bound Con A was associated with those nanoscale features.