A protein partially expressed on the surface of HepG2 cells that binds lipoproteins specifically is nucleolin

Abstract

Nucleolin, a major nucleolar protein of rapidly growing eukaryotic cells, has been thought to be predominantly if not exclusively located in the nucleolus. Recent data however [Borer, R. A., Lehner, C. F., Eppenberger, H. M., and Nigg, N. A. (1989) Cell 56, 379-390] suggest that the protein shuttles constantly between the nucleus and cytoplasm. Ligand blotting studies of whole cell extracts of HepG2 cells identified, in addition to the LDL receptor, another LDL binding protein of Mr 109,000. The 109-kDa protein was partially purified by HPLC and, like the LDL receptor, bound apoB- and apoE-containing lipoproteins but not HDL. However, unlike the LDL receptor, the 109-kDa protein bound lipoproteins in the presence of EDTA and reducing agents, had a lower affinity for lipoproteins than the LDL receptor, and did not react with two antibodies raised against the LDL receptor. The protein sequences of three separate peptides derived from the partially purified 109-kDa species were determined and were identical except for one residue to three separate regions of the published sequence of nucleolin. On immunoblot analysis the 109-kDa protein reacted with a nucleolin-specific antibody, and purified nucleolin reacted both with anti-109-kDa antibody and with LDL. When intact HepG2 cells were treated with Pronase before harvest, there was a 46% decrease in 100-kDa protein while recovery of actin, an intracellular protein, was unaffected. When intact HepG2 cells were surface iodinated and the proteins subjected to HPLC fractionation, the 109-kDa protein was found to be iodinated. On blots, the binding of 125I-LDL to the 109-kDa protein was inhibited by anti-109-kDa antibody and by the nucleolin-specific antibody. In addition, anti-109-kDa antibody significantly decreased the specific binding of 125I-LDL to intact HepG2 cells. These data suggest that the lipoprotein-binding 109-kDa protein is nucleolin-like protein that is partially expressed on the cell surface.