tw9 related proteins, transferrin and lactoferrin. Transferrin serves as the principal chelator of extracellular iron in serum and transferrin receptors playa major role in cellular iron uptake through the carrier protein transferrin. Till date, two receptors for transferrin (TfRl

Iron is the most common element on the earth and is crucial for the growth of all organisms. Despite having a crucial role in cellular physiology, excess of iron leads to the generation of reactive free radicals that are highly toxic to cells. It is therefore vital for cells to maintain a perfect balance of intracellular iron. Iron acquisition, transport & storage processes are therefore very tightly regulated in mammalian cells. Serum iron is mostly chelated to one of tw9 related proteins, transferrin and lactoferrin. Transferrin serves as the principal chelator of extracellular iron in serum and transferrin receptors playa major role in cellular iron uptake through the carrier protein transferrin. Till date, two receptors for transferrin (TfRl & TOO) have been identified in mammalian systems. TfRl mediated endocytosis of Tf is the major pathway for iron uptake while the role of TOO in iron acquisition is still unclear. Although TfTfRl mediated endocytosis has been well characterized for iron uptake there are indications that additional transferrin receptors also exist and contribute to meet the iron requirement of cells for survival. Previous studies from our laboratory have reported the presence of a third receptor for transferrin. Interestingly, it was the multifunctional glycolytic enzyme GAPDH. Various studies have indicated that GAPDH functions not only as glycolytic enzyme but exhibits numerous diverse biological properties unrelated to its glycolytic function. This is true for both prokaryotes and eukaryotes. GAPDH, which bears no homology to either of the two known transferrin receptors, was found to be expressed on cell surface of macrophage cells where it functions as a, previously unknown, transferrin receptor. The affmity of GAPDH for Tf was found to be approximately 60-100 fold lower than TfRl. This suggests that although TfR1 may be the major pathway of transferrin mediated iron uptake, cell surface GAPDH could function as lower affmity receptor for Tf. The existence of such a low affmity pathway for Tf uptake has earlier been speculated upon in literature. The present study was aimed at the detailed characterization of this cell surface GAPDH as an additional transferrin receptor and its role in iron acquisition. The first question that we looked at was, if there is any difference between cytosolic and cell surface GAPDH so as to explain its membrane localization. We were unable to detect any difference in both forms of GAPDH in terms of; N-terminal sequence, Peptide mass fmgerprinting and PAS staining (glycosylation). In silico analysis of the GAPDH sequence also could not provide any clues regarding any possible post translational modifications that can result in its recruitment to cell membrane. Interestingly, intact mass MALDITOF analysis of cytosolic and membrane GAPDH revealed a significant