The early response by lymphocytes to challenge with cell surface-directed ligand, such as antibody or concanavalin A (Con A), involves the clustering of initially diffuse ligand-;receptor complexes into patches, followed by the gathering of these patches into a cap over one region of the cell and their endocytosis1-;3. Using cells of the human lymphoblastoid line WiL2, we have previously shown that receptor-mediated endocytosis occurring at caps involves clathrin-coated vesicles and that the formation of such vesicles is sensitive to drugs such as trifluoperazine dihydrochloride (TFP) that affect calmodulin (CaM), the calcium-dependent regulatory protein of cells3. By indirect immunofluorescence, we demonstrate here that in WiL2, before surface-directed ligand challenge, CaM is distributed diffusely in the cell. On challenge, concurrent with capping of cell-surface receptors for Con A, CaM localization becomes concentrated in regions of the cytoplasm just below the cap. We show that CaM redistribution is sensitive to TFP and dependent on Ca2+ in the external milieu. Capping is mostly unaffected by TFP or removal of external Ca2+. Cytochalasin D (CD), on the other hand, blocks capping completely, but does not prevent the initial redistribution of CaM. These results are consistent with a functional role for CaM in clathrin recruitment to the cell surface beneath ligand-;receptor complexes and an initial Ca2+ requiring, microfilament-independent event in receptor-mediated endocytosis.
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