Cellular distribution of high and low affinity concanavalin A binding sites on the plasma membrane of normal human fibroblasts.

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Abstract
There are a few high affinity binding sites for concanavalin A [ConA] on the plasma membrane of normal human fibroblasts. To determine if these high affinity sites are distributed uniformly on all cells or are present in high concentrations on only a small percentage of the total cell population, immunoelectron microscopy was used to observe the topological distribution of Con A on the cell surface. A uniform distribution of binding sites was found when the cells were incubated with 50 .mu.g/ml con A, a concentration sufficient to saturate low and high affinity sites, or 0.1 .mu.g/ml con A, a concentration sufficient to saturate only high affinity sites. Binding of con A was not confined to any localized or specialized regions of the membrane. Autoradiography and fluorescent microscopy techniques were used to further examine con A binding sites on the plasma membranes of individual cells. All cells showed a uniform distribution of con A when high and low affinity sites were saturated. When only high affinity sites were saturated, absence of any detectable binding by fluorescence or autoradiography set quantitative lower limits on possible levels of nonuniform binding. Apparently there is no significant degree of nonuniform distribution of con A binding sites on the surface of a single cell or among total population of cells. If any asymmetry exists, it must involve more than 15% of the total cell population.