We have characterized a cDNA encoding a cysteine-rich, acidic integral membrane protein (CRAM) of the parasitic protozoa Trypanosoma brucei and Trypanosoma equiperdum. Unlike other membrane proteins of T. brucei, which are distributed throughout the cell surface, CRAM is concentrated in the flagellar pocket, an invagination of the cell surface of the trypanosome where endocytosis has been documented. Accordingly, CRAM also locates to vesicles located underneath the pocket, providing evidence of its internalization. CRAM has a predicted molecular mass of 130 kilodaltons and has a signal peptide, a transmembrane domain, and a 41-amino-acid cytoplasmic extension. A characteristic feature of CRAM is a large extracellular domain with a roughly 66-fold acidic, cysteine-rich 12-amino-acid repeat. CRAM is conserved among different protozoan species, including Trypanosoma cruzi, and CRAM has structural similarities with eucaryotic cell surface receptors. The most striking homology of CRAM is to the human low-density-lipoprotein receptor. We propose that CRAM functions as a cell surface receptor of different trypanosome species.The protozoan parasite Trypanosoma brucei, causative agent of sleeping sickness, survives in blood by undergoing antigenic variation of its variant cell surface glycoprotein (VSG) coat. The parasite is transmitted by the tsetse fly through ingestion in a bloodmeal and transfer with the tsetse fly saliva. In the tsetse fly, the bloodstream form trypanosomes differentiate into insect (procyclic) form trypanosomes, resulting in the loss of the protective VSG coat and the expression of a different and small family of cell surface proteins, the procyclic acidic repetitive proteins (or procyclins; 6, 7, 39, 44, 45, 55, 59). Both VSG and the procyclic acidic repetitive proteins are extracellular proteins anchored to the lipid bilayer by a covalently attached lipid moiety (12,15,43). Proteins with transmembrane domains have not been characterized in trypanosomes thus far.Previous studies led to the proposal that a low-densitylipoprotein (LDL) receptor, a transferrin receptor, a nucleoside transport receptor, and a growth factor receptor exist in T. brucei (13,14,26; G. Hide, A. Gray, C. M. Harrison, and A. Tait, Mol. Biochem. Parasitol., in press). A dense network of microtubules below the plasma membrane presumably prevents the uptake of cell surface receptors and their ligands through endocytosis. However, the flagellar pocket, a deep invagination of the plasma membrane where the flagellum extends from the cell, is not lined with this subpellicular microtubule network. The flagellar pocket is believed to be the site of uptake of nutrients and macromolecules, possibly via receptor-mediated endocytosis (13,14,32,40,57,58,59,62). Here we characterize a conserved transmembrane protein localized in the flagellar pocket of T.