THE ACTIN CYTOSKELETON IS REQUIRED FOR RECEPTOR-MEDIATED ENDOCYTOSIS IN MAMMALIAN-CELLS

Actin filament organization is essential for endocytosis in yeast. In contrast, the actin-depolymerizing agent cytochalasin D has yielded am biguous results as to a role for actin in receptor mediated endocytosi s in mammalian cells. We have therefore re-examined this issue using h ighly specific reagents known to sequester actin monomers. Two of thes e reagents, thymosin beta 4 and DNase I, potently inhibited the seques tration of transferrin receptors into coated pits as measured in a cel l-free system using perforated A431 cells. At low concentrations, thym osin beta 4 but not DNase I was stimulatory, Importantly, the effects of both reagents were specifically neutralized by the addition of acti n monomers. A role for the actin cytoskeleton was also detected in int act cells where latrunculin A, a drug that sequesters actin monomers, inhibited receptor-mediated endocytosis. Biochemical and morphological analyses suggest that these reagents inhibit later events in coated v esicle budding. These results provide new evidence that the actin cyto skeleton is required for receptor-mediated endocytosis in mammalian ce lls.