title: Direct visualization of the binding and internalization of a ferritin conjugate of epidermal growth factor in human carcinoma cells A-431.
abstract: We have prepared a conjugate of epidermal growth factor (EGF) and ferritin that retains substantial binding affinity for cell receptors and is biologically active. Glutaraldehyde-activated EGF was covalently linked to ferritin to produce a conjugate that contained EGF and ferritin in a 1:1 molar ratio. The conjugate was separated from free ferritin by affinity chromatography using antibodies to EGF. Monolayers of human epithelioid carcinoma cells (A-431) were incubated with EGF:ferritin at 4 degrees C and processed for transmission electron microscopy. Under these conditions, approximately 6 X 10(5) molecules of EGF:ferritin bound to the plasma membrane of each cell. In the presence of excess native EGF, the number of bound ferritin particles was reduced by 99%, indicating that EGF:ferritin binds specifically to cellular EGF receptors. At 37 degrees C, cell-bound EGF:ferritin rapidly redistributed in the plane of the plasma membrane to form small groups that were subsequently internalized into pinocytic vesicles. By 2.5 min at 37 degrees C, 32% of the cell-bound EGF:ferritin was localized in vesicles. After 2.5 min, there was a decrease in the proportion of conjugate in vesicles with a concomitant accumulation of EGF:ferritin in multivesicular bodies. By 30 min, 84% of the conjugate was located in structures morphologically identified as multivesicular bodies or lysosomes. These results are consistent with other morphological and biochemical studies utilizing 125I-EGF and fluorescein-conjugated EGF.
mesh_terms: D002294:Carcinoma, Squamous Cell; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D004815:Epidermal Growth Factor / Q000378:metabolism*; D005293:Ferritins / Q000378:metabolism*; D006801:Humans; D010455:Peptides / Q000378:metabolism*; D010873:Pinocytosis; D011955:Receptors, Drug / Q000378:metabolism*; D013696:Temperature
chemical_list: D010455:Peptides; D011955:Receptors, Drug; D004815:Epidermal Growth Factor; D005293:Ferritins