Low affinity of Trypanosoma brucei transferrin receptor to apotransferrin at pH 5 explains the fate of the ligand during endocytosis

Abstract

~,bstract Uptake of host transferrin (Tf) in Trypanosoma I,rucei is mediated by a heterodimeric, glycosyl-phosphatidylino~itol-anchored receptor. After endocytosis, Tf is delivered to lysosomes where it is proteolytically degraded. So far, the -equence of events leading to ligand dissociation and degradation is undefined. We now show by Triton X-114 phase separation I hat iron-free Tf (apo-Tf) dissociates from the receptor at pH 5.0. The low affinity of apo-Tf for its receptor at pH 5.0 is confirmed by an apparent dissociation constant of 1.1 pM. The implications of this result on the mechanism of intracellular Fwocessing of Tf in trypanosomes are discussed. ~2ey words." Transferrin receptor; Transferrin; erypanosoma brucei • IntroductionThe transferrin receptor (TfR) of Trypanosoma brucei difers in primary structure, subunit organisation and mode of nembrane anchorage from the mammalian TfR. The trypalosome TfR is a heterodimeric complex of very low abunlance (about 3000 molecules/cell) encoded by two expression rite-associated genes (ESAGs), ESAG6 and ESAG7 [1][2][3][4][5]. The ESAG6 product (pESAG6) is a heterogeneously glycosylated 9rotein of 50-60 kDa modified by a glycosyl-phosphatidylnositol membrane anchor, while the ESAG7 product (pEgAG7) is a 42 kDa glycoprotein carrying an unmodified COOH-terminus [1]. Binding of one molecule of transferrin Tf) [2] requires association of both pESAG6 and pESAG7 as ~hown by coexpression in heterologous systems [3][4][5]. Despite :he profound difference in receptor structure, the apparent tissociation constant (Kd) for iron-loaded Tf (holo-Tf) is of ::he same order of magnitude for both the trypanosome TfR 3.6-108 nM [2]) and the mammalian TfR (2-110 nM [6,7]). Fhe fate of Tf differs, however, in mammalian cells and try3anosomes. In mammalian cells, the Tf-TfR complex is transported to endosomes where Tf releases its iron. The iron-free l'f (apo-Tf) remains bound to its receptor and is recycled back ~.o the cell surface where it dissociates from the receptor [6,8]. In contrast, Tf is delivered in trypanosomes to lysosomes ~vhere it is proteolytically degraded [2,9]. While the degradation products are released from the cells, iron remains cell associated [2]. As the sequence of events leading to ligand clissociation and degradation in T. brucei is undefined, we have studied the affinity of Tf for the trypanosome TfR at different pH values by Triton X-114 phase separation [10] and by determination of Ka values using membranes of trypanosomes. The results allow us to explain the intracellular processing of Tf in trypanosomes as compared to that of mammalian cells. Materials and methods ReagentsBovine holo-transferrin (bolo-Tf), N-dodecyl-N,N-dimethyl-3-ammonio-l-propanesulfonate (lauryl sulfobetaine) and p-chloromercuribenzenesulfonic acid (PCMBS) were purchased from Sigma, Deisenhofen, Germany; Triton X-114 from Serva, Heidelberg, Germany; and sodium boro [aH]hydride (31 Ci/mmol) from Amersham, Braunschweig, Germany. TrypanosomesVariant clone MITat 1.4 of 72 brucei strain...

Iron metabolism in trypanosomatids, and its crucial role in infection

“…Unlike the mammalian transferrin receptor, which remains tightly bound to its cargo throughout, ESAG6/7 loses affinity for apotransferrin at low pH. The apotransferrin is then degraded by the cathepsin B-like protein, TbcatB in the lysosome (Maier and Steverding, 1996 ;O'Brien et al 2008), while the receptor is recycled back to the flagellar pocket via TbRab11 positive vesicles (Steverding et al 1995 ;Jeffries et al 2001). The degraded transferrin fragments are recycled to the surface and exocytosed, also by a Rab11-dependent mechanism (Pal et al 2003 ;Hall et al 2005).…”

Bloodstream formTrypanosoma bruceidepend upon multiple metacaspases associated with RAB11-positive endosomes

“…Following internalisation of the receptor-transferrin complex, the reduction in pH within the endosomal system is believed to cause dissociation of transferrin from the receptor (DautryVarsat et al, 1983;Maier and Steverding, 1996). The transferrin receptor is then recycled back to the flagellar pocket by Rab11-positive recycling endosomes (Jeffries et al, 2001) and degraded transferrin is secreted (Steverding et al, 1995;Pal et al, 2003).…”

Acetaldehyde/alcohol dehydrogenase-2 (EhADH2) and clathrin are involved in internalization of human transferrin by Entamoeba histolytica

“…The presence of clathrin has been demonstrated in several protozoa (Corrêa et al, 2007;Hernandez et al, 2007;Morgan et al, 2001;Robibaro et al, 2001) (Maier & Steverding, 1996).…”

“…Apotransferrin (apoTf, Tf without iron) is thus formed and, still bound to the TfR, is sorted into exocytic vesicles that fuse with the plasma membrane. The complex is then exposed to the extracellular pH, where apoTf has a very low affinity for the TfR and is thus dissociated from it, ready for another cycle of iron capture and endocytosis (Dautry-Varsat et al, 1983;Thorstensen & Romslo, 1990).The presence of clathrin has been demonstrated in several protozoa (Corrêa et al, 2007;Hernandez et al, 2007;Morgan et al, 2001;Robibaro et al, 2001) (Maier & Steverding, 1996).E. histolytica causes amoebiasis, a disease characterized by dysentery and abscesses in the large intestine and liver.…”