Biogenesis of microsomal membrane glycoproteins in rat liver. IV. Characteristics of a cytoplasmic lipoprotein having properties of a membrane precursor.

The supernatant fraction from a rat liver homogenate contains a lipoprotein complex which can be incorporated into microsomal membranes in vitro. The lipoprotein can be purified by gel filtration and flotation in a NaBr solution. The isolated lipoprotein has an equilibrium density in the range of 1.07-1.14 g/ml, a sedimentation coefficient of 4.9 S and a calculated molecular weight of 210 000. The isolated complex contains 44% lipid by weight, half of which is phospholipid. The protein also has covalently bound sugar moieties, namely mannose, galactose, glucosamine, and sialic acid. The complex is unstable and may dissociate during the isolation procedure. The complex and its fragments contain two peptides with molecular weights of 11-13000 and 67-69000. These two components have practically identical amino acid compositions.