Time-dependent lectin binding to isolated receptors in model membranes

Binding of lectins to two integral membrane glycoproteins has been measured in lipid bilayer model membranes and in their cell of origin with an eye to clarifying the basis of time-dependence in such processes. Specific binding was monitored as a function of time using an assay that involved membrane exposure to radiolabelled wheat-germ agglutinin or concanavalin A, and subsequent differential centrifugation to remove unbound material. Qualitatively, the time dependence of lectin binding to the isolated receptors in lipid bilayers was found to be similar to that for the same receptors in the intact cell - hence the phenomenon does not depend for its existence upon receptor interaction with other specific native membrane components. Quantitatively, the time-course was sensitive to structural features of the model membrane involved. The results may be understood by viewing the glycopeptide headgroups as deformable structures which rearrange as a direct result of lectin attachment, and it would appear that rearrangement is essential for high-affinity binding. Model membrane structure was examined by light microscopy and freeze-etch electron microscopy in an attempt to assess the applicability of this type of study to a more detailed analysis of the processes involved in lectin binding. Although the freeze-etch technique is a promising one, it was concluded that heterogeneity in receptor arrangement within the lipid bilayer is the most important limitation to correlation of binding curves with membrane structure.