Receptors for the host low density lipoproteins on the hemoflagellate Trypanosoma brucei: Purification and involvement in the growth of the parasite

The slender bloodstream form of Trypanosoma brucei shows receptor-mediated endocytosis of low density lipoprotein (LDL) particles of its hosts. We have purified the LDL receptor of this species nearly to homogeneity (about 1000-fold purification) and obtained monospecific polyclonal antibodies against it. As analyzed by NaDodSO₄/polyacrylamide gel electrophoresis, the purified receptor consists of a single subunit, with an apparent molecular mass of 86 kDa. Its isoelectric point is 5.9. On the average, each cell exposes 52,000 copies of low-affinity receptors (K(d) of 250 nm) and 1800 copies of high-affinity receptors (K(d) of 5.7 nM). According to indirect en bloc immunolabeling of fixed parasites, the receptor appears to be localized to the flagellar pocket membrane and the flagellar membrane and to be completely absent from the rest of the pericellular membrane. LDL is required for optimal growth of the trypanosome in vitro: cell growth can be inhibited either by removal of LDL from the culture medium or by antibodies against the purified LDL receptors. In both cases, growth is restored by the addition of excess LDL.