The multifunctional glycolytic protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a novel macrophage lactoferrin receptor.

Several proteins with limited cell type distribution have been shown to bind lactoferrin. However, except in the case of hepatic and intestinal cells, these have not been definitively identified and characterized. Here we report that the multifunctional glycolytic protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) functions as a novel receptor for lactoferrin (Lf) in macrophages. GAPDH is a well-known moonlighting protein, and previous work from our laboratory has indicated its localization on macrophage cell surfaces, wherein it functions as a transferrin (Tf) receptor. The K(D) value for GAPDH-lactoferrin interaction was determined to be 43.8 nmol/L. Utilizing co-immunoprecipitation, immunoflorescence, and immunogold labelling electron microscopy we could demonstrate the trafficking of lactoferrin to the endosomal compartment along with GAPDH. We also found that upon iron depletion the binding of lactoferrin to macrophage cell surface is enhanced. This correlated with an increased expression of surface GAPDH, while other known lactoferrin receptors CD14 and lipoprotein receptor-related protein (LRP) were found to remain unaltered in expression levels. This suggests that upon iron depletion, cells prefer to use GAPDH to acquire lactoferrin. As GAPDH is an ubiquitously expressed molecule, its function as a receptor for lactoferrin may not be limited to macrophages.