Specific postendocytic proteolysis of apolipoprotein B in oocytes does not abolish receptor recognition

Upon receptor-mediated transfer of plasma very low density lipoprotein (VLDL) particles into growing chicken oocytes, their major apolipoprotein (apo) component, apoB, is proteolytically cleaved. apoB fragmentation appears to be catalyzed by cathepsin D or a similar pepstatin A-sensitive protease and results in the presence of a characteristic set of polypeptides on yolk VLDL particles. The nicks introduced into the apoB backbone during postendocytic processing occur in yolk platelets and appear to prepare internalized VLDL for storage in yolk. Since yolk VLDL binds to chicken receptors specific for apoB-containing lipoproteins in identical fashion to plasma VLDL, the possibility exists that the developing embryo utilizes yolk VLDL as a nutrient by way of receptor-mediated endocytosis.