Receptor-mediated entry of Pseudomonas toxin: methylamine blocks clustering step.

Clustering of ligands into coated regions of the plasma membrane is an early step in receptor-mediated endocytosis. The association of Pseudomonas exotoxin A (PE) with mouse LM fibroblasts was visualized by using biotinyl-PE and avidingold. Movement of PE into coated regions occurred within 30 s of warming monolayers to 37 degrees C. This clustering was stopped by the primary amines methylamine and ammonium chloride but was not altered by the tertiary amine chloroquine. Toxin internalization was rapid, with a half-time of approximately 5 min. Although primary amines stopped clustering, they did not alter the rate of toxin internalization; they did alter the route followed after entry. We have shown previously that methylamine protects cells from the lethal action of PE. Here we suggest that methylamine protects, at least in part, by blocking clustering, and that receptor-mediated endocytosis is required for efficient expression of PE toxicity.