Identification of a circulatory and oocytic avian apolipoprotein D

Apolipoprotein D (apo D) is an unusual apolipoprotein with respect to structure and sites of synthesis. It has been identified in the circulatory system of certain mammals, but its physiological role remains poorly understood. In this report, it is shown that apo D is not exclusively a mammalian apolipoprotein, and evidence is presented which suggests a novel function for this protein during oogenesis in the chicken. The avian apo D which we identify has the same molecular mass (29 kDa) as the human protein and also associates preferentially with the plasma lipoprotein fraction. In addition to the 29 kDa avian apo D species, an immunoreactive 24 kDa protein is observed in chicken serum. The chicken apo D (along with the 24 kDa species) is also demonstrated to be present in the yolk of the rapidly growing chicken oocyte, a cell with high endocytic activity. Clathrin-coated vesicles from chicken oocytes, which we have previously shown to contain specific lipoproteins along with their oocytic receptors (Bujo et al., 1994: EMBO J 13:5165-5175), also contain chicken apo D. Thus, apo D represents a novel candidate for plasma-to-oocyte transport of lipids and/or their mobilization during embryogenesis in oviparous species.