JBC, Vol. 250, Issue 17, 6706-6710, Sep, 1975
Studies of the cell surface of Dictyostelium discoideum during differentiation. The binding of 125I-concanavalin A to the cell surface
G. Weeks
125I-concanavalin A (125I-Con A) was found to be equally effective as
native Con A in binding to and agglutinating cells of Dictyostelium
discoideum, suggesting that iodination of the molecule had no effect on the
interaction of the protein with the cell surface. Almost all of the
125I-Con A binding to the cells was inhibited by alpha-methyl glucoside.
The binding of 125I-Con A to the cells was extremely rapid, and once bound,
the molecule was not readily displaced by prolonged incubation or by the
addition of excess native concanavalin A (Con A). In contrast, the 125I-Con
A was displaced rapidly from the cell surface by alpha-methyl glucoside.
The binding of 125I-Con A to D. discoideum was identical at 22 degrees and
4 degrees, and was unaffected by metabolic inhibitors, suggesting that the
protein was not subject to endocytosis. The cell surface Con A binding
sites became saturated at high 125I-Con A concentrations. Scatchard plots
of the data indicated that growing cells possessed 4 X 10(7) sites/cell,
all of equal affinity. Similar plots for "aggregation phase" cells
indicated at least two classes of binding sites. A small proportion of the
sites had an affinity close to that for the sites on growing cells, but the
majority of the sites had a markedly decreased affinity. The total number
of binding sites increased only slightly during aggregation to 5.6 X 10(7)
sites/cell.
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