Avian riboflavin binding protein binds to lipoprotein receptors in association with vitellogenin

Riboflavin binding protein (ribBP) is an essential component of chicken eggs; it supplies the oocyte (i.e. yolk) and egg white with sufficient amounts of the vitamin riboflavin to sustain embryonic development until hatching. There are three forms of ribBP in the laying hen; synthesized by the liver under the control of estrogen, it enters the serum (sribBP) and is delivered to yolk where it becomes carboxyl-terminally truncated (yribBP). The egg white form (wribBP), synthesized by the oviduct, is a product of the same gene as sribBP but has a different glycosylation pattern. Our efforts to delineate the mechanism for uptake of yolk precursors into rapidly growing chicken oocytes have previously identified a multifunctional oocyte-specific 95-kDa lipoprotein receptor that belongs to the low density lipoprotein receptor gene family (Barber, D.L., Sanders, E.J., Aebersold, R., and Schneider, W.J. (1991) J. Biol. Chem. 266, 18761-18770). We now report that in serum, ribBP associates with another yolk precursor, the lipid-, phosphate-, and ion-carrier vitellogenin (VTG), a known ligand of the 95-kDa receptor. In the presence of VTG, 125I-labeled sribBP binds to the 95-kDa receptor and, under certain conditions, also to the avian oocyte low density lipoprotein receptor-related protein (Stifani, S., Barber, D.L., Aebersold, R., Steyrer, E., Shen, X., Nimpf, J., and Schneider, W.J. (1991) J. Biol. Chem. 266, 19079-19087). The interaction between ribBP and the 95-kDa receptor and/or VTG requires Ca2+ and PO4(3-). Interestingly, sribBP, yribBP, and wribBP are all capable of VTG-associated receptor binding. This demonstrates that (i) the carboxyl-terminal 11 or 13 amino acids, which are removed from sribBP upon oocytic uptake, are not involved in receptor binding and (ii) receptor binding and/or association of ribBP with VTG is not dependent on the carbohydrate structure present on sribBP. The results indicate that the oocytic uptake of sribBP is mediated, through association with VTG, by the 95-kDa receptor and possibly other oocytic members of the low density lipoprotein receptor gene family, adding an interesting and novel variation to ligand recognition by these receptors.