BOVINE SERUM-ALBUMIN IS A MAJOR OLIGONUCLEOTIDE-BINDING PROTEIN FOUNDON THE SURFACE OF CULTURED-CELLS

To better understand the uptake of oligonucleotides into cells, we hav e studied the labeling of cell surface proteins by an oligonucleotide conjugated to a radiolabeled photoactivatable crosslinker (Denny-Jaffe reagent). When HL60 cells are treated with the conjugate for 2 hours in a medium containing bovine serum albumin (BSA), almost all of the c ell-associated label is found in one protein, which we identify as BSA . Cells grown and treated in a serum-free medium do not show this prot ein, whereas it is plainly seen in cells that are grown in serum-conta ining medium but then treated in serum-free medium. Overall associatio n of the oligonucleotide with cells is much higher in serum-free mediu m than in BSA-containing medium, but the oligonucleotide is mostly not protein-associated in the absence of BSA. We conclude that (1) BSA fr om the medium serves to block overall association of oligonucleotide w ith cells, and (2) BSA is the main cell surface protein binding oligon ucleotides. We discuss the possible role of albumin in endocytic uptak e of oligonucleotides in the cell and in the biodistribution of oligon ucleotides in vivo.