ALTERNATE IRON TRANSPORT PATHWAY - MOBILFERRIN AND INTEGRIN IN K562 CELLS

A transferrin-independent iron transport system in cells containing tr ansferrin receptors was described previously by several investigators. Prior studies did not identify the proteins involved in this alternat e iron transport pathway. Using a human-derived erythroleukemia tissue culture line, iron-binding proteins were isolated from cytosol and ce ll membranes. The cytosol protein was soluble in 60% ammonium sulfate, had a molecular mass similar to mobilferrin (56 kDa), and reacted wit h anti-mobilferrin antibodies. The water-in soluble radiolabeled prote in was solubilized with Nonidet P-40 and immunoprecipitated with monoc lonal antibody against beta 3 human integrin. Pulse-chase studies sugg ested sequential passage of iron to integrin, mobilferrin, and ferriti n, respectively. Thus, the alternate iron transport pathway contained proteins similar to those observed in intestinal cells which did not p ossess transferrin receptors on their absorptive surface. The alternat e iron transport pathway is only partially shared with zinc and cadmiu m. Mobilferrin bound zinc and iron competitively, but the two metals w ere not transported competitively into K562 cells. Immunoprecipitates of integrin containing radiozinc were obtained with a monoclonal antib ody against beta 1 human integrin. This suggested iron and zinc may ut ilize different integrins to passage the cell membrane.