Expression and secretion of chicken apolipoprotein AI in transfected COS cells

A full-length chicken apolipoprotein A-I (apoAI) cDNA has been cloned into an expression vector, pRSVapoAI. This plasmid was transfected into a monkey kidney (COS-1) cell line in order to study apolipoprotein-lipid assembly. Chicken apoAI is the major apolipoprotein of chicken high-density lipoprotein (HDL), which is less complex in apolipoprotein content than the HDL of human plasma. The transient transfected COS-1 cells synthesized and secreted authentic plasma apoAI. Under serum-free medium conditions, COS cells secreted only proapoAI. A small portion (15%) of the secreted apoAI floated at a density 1.07-1.20 g/ml. Upon incubation with fetal bovine serum at 10 degrees C, a majority of the apoAI was recovered in the HDL density (1.06-1.20 g/ml) region. Secreted apoAI was labeled when transfected COS cells were incubated with U-14C]palmitate, but the incorporation of radioactivity was not the result of fatty acid acylation through ester bond formation. These results indicate that heterologous COS-1 cells are capable of synthesizing and secreting apoAI, and that intracellular association of apoAI with lipids is not necessary for secretion.