The chicken oocyte receptor for lipoprotein deposition recognizes alpha 2-macroglobulin

alpha 2-Macroglobulin (alpha 2M), a major plasma component in all vertebrates, is proposed to function as a broad spectrum protease inhibitor. The alpha 2M-proteinase complex (activated alpha 2M; alpha 2M*) is removed rapidly by receptor-mediated endocytosis in the liver. Here we demonstrate by Western blotting that alpha 2M is also present in the yolk of chicken oocytes. Plasma levels of alpha 2M are increased by estrogen, and yolk alpha 2M is partially proteolyzed, consistent with the action of cathepsin D on endocytosed alpha 2M. Two known estrogen-induced ligands of the oocyte-specific 95-kDa very low density lipoprotein/vitellogenin receptor (OVR) are also fragmented by yolk cathepsin D (Retzek, H., Steyrer, E., Sanders, E. J., Nimpf, J., and Schneider, W. J. (1992) DNA Cell Biol. 11, 661-672). Since these findings suggested a common uptake mechanism for lipoproteins and alpha 2M by oocytes, we investigated whether OVR, a member of the low density lipoprotein receptor family, functions in the metabolism of alpha 2M. Ligand blotting of oocyte membrane extracts with chicken alpha 2M* revealed that it binds to OVR. Surprisingly, the oocyte receptor also recognizes native alpha 2M, in sharp contrast to the hepatic receptor, which only binds alpha 2M*. Receptor interaction of both forms requires Ca2+; however, competition experiments suggest that alpha 2M and alpha 2M* interact with slightly different, or overlapping, sites on the receptor. Colocalization of alpha 2M and OVR in coated vesicles isolated from growing oocytes, and internalization and degradation of methylamine-activated alpha 2M by COS-7 cells transfected with OVR, strongly suggest that alpha 2M is transported into growing oocytes via OVR. We propose that this multifunctional receptor mediates pathways at the metabolic crossroads of lipoproteins and protease inhibitor complexes.