RECEPTOR-ASSOCIATED PROTEIN IN AN OVIPAROUS SPECIES IS CORRELATED WITH THE EXPRESSION OF A RECEPTOR VARIANT

The biosynthesis of proteins containing cysteine-rich domains requires chaperones for their correct folding. For instance, the 39-kDa recept or-associated protein (RAP) aides in the cell-surface targeting of new ly synthesized members of the mammalian low density lipoprotein recept or (LDLR) gene family, which contains tandemly arranged clusters of he xacysteine repeats. In the chicken, an LDLR relative with eight such r epeats is expressed as two different splice variant forms in cell type -specific fashion (Bujo, H., Lindstedt, K. A., Hermann, M., Mola Dalma u, L., Nimpf, J., and Schneider, W. J. (1995) J. Biol. Chem. 270, 2354 6-23551). To learn more about evolutionary aspects of RAP, its role in escorting of these different receptor splice variants, and other pote ntial functions, we have extended our studies on the avian LDLR family to RAP. cDNA cloning, determination of tissue expression at both the transcript and the protein level, stable expression in COS cells, and binding studies with chicken RAP revealed that mammalian RAPs have ret ained many features of the nonamniotic proteins. However, structural d etails, e.g. the well defined internal triplicate repeats in the chick en protein, have been somewhat diluted during evolution. Interestingly , chicken RAP was found to correlate positively with the expression le vels in somatic cells of the larger splice variant of the eight-cystei ne repeat receptor, but not with those of the smaller variant, express ed only in germ cells. This is compatible with the possibility that RA P may play a role in receptor biology that could be complementing its function in assisting folding. Chicken RAP in crude extracts of the st able expressor COS cells is able to bind to LDLR relatives in ligand b lots without requirement for prior purification of the ligand. Thus, i n conjunction with the avian model of massive lipid transport to germ cells, these cells provide a novel comparative system amenable to inve stigation of the biological functions of RAP.