The effects of isolated plasma membranes on cohesion of Dictyostelium discoideum Ax-2 cells have been studied. Membranes isolated from cells in the log phase of growth gave complete inhibition of log-phase cell cohesion. This effect was specific for the cohesion log-phase cells mediated by contact sites B, since the cohesion of aggregation-competent cells which have acquired contact sites A was only partially inhibited by log-phase plasma membranes. Membranes isolated from stationary phase cells gave partial inhibition of log-phase cell cohesion, while membranes from aggregation-competent cells gave complete inhibition of log-phase cell cohesion but at much higher concentration than log membranes. Treatment of log-phase cells with cycloheximide for 8 h rendered them completely non-cohesive. Membranes from cycloheximide-treated cells had no effect on log-phase cell cohesion. Log-phase membranes gave complete inhibition of cohesion of 4 slime mould species. The results are discussed in terms of our ligand-receptor model of log-phase cell cohesion.

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