title: GPI valence and the fate of secretory membrane proteins in African trypanosomes.
abstract: Progression of GPI-anchored proteins in bloodstream African trypanosomes correlates with GPI-valence: homodimeric VSG (2 GPI) is a surface protein; heterodimeric transferrin receptor (1 GPI) localizes in the flagellar pocket; homodimeric GPI-minus VSG (0 GPI) is rapidly degraded in the lysosome. We test this relationship using three native secretory/endocytic proteins as monomeric GPI-plus and -minus reporters. GPI-minus procyclin trafficks to the lysosome and is degraded. GPI-plus procyclin trafficks to the flagellar pocket/cell surface and is released (approximately 50%) with an intact anchor, the remainder (approximately 50%) is degraded in the lysosome. GPI-plus BiPNHP, derived from the ER marker BiP, is released quantitatively (>80%), while GPI-plus p67HP, derived from the lysosomal marker p67, turns over by both release (approximately 15%) and lysosomal degradation (>50%). Turnover of endogenous transferrin receptor occurs primarily by lysosomal degradation (>90%). Thus shedding of monovalent GPI reporters correlates inversely with lysosomal targeting. We propose that mono-GPI reporters cycle through the flagellar pocket and endosome until they are disposed of by either shedding or lysosomal targeting. Partitioning between these fates may be a function of individual physical properties. Release is likely due to the exclusive use of C-14:0 myristate in the bloodstream stage GPI anchor. Up-regulation of transferrin receptor by culture in dog serum resulted in prominent cell surface localization, but not in elevated release. Surface receptor was non-functional for ligand binding suggesting that it may be bivalent homodimers of the GPI-anchored ESAG6 receptor subunit.
authors.affiliation: Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Medical School, Madison, WI 53706, USA.
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Schwartz
authors.forename: Ronald F
authors.initials: RF
authors.lastname: Peck
authors.forename: Ngii N
authors.initials: NN
authors.lastname: Tazeh
authors.forename: James D
authors.initials: JD
authors.lastname: Bangs
mesh_terms: D000818:Animals; D017261:Glycosylphosphatidylinositols / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism; D011990:Receptors, Transferrin / Q000378:metabolism; D014346:Trypanosoma brucei brucei / Q000235:genetics / Q000378:metabolism*; D014643:Variant Surface Glycoproteins, Trypanosoma / Q000235:genetics / Q000378:metabolism*
chemical_list: D017261:Glycosylphosphatidylinositols; D008562:Membrane Glycoproteins; D015800:Protozoan Proteins; D011990:Receptors, Transferrin; D014643:Variant Surface Glycoproteins, Trypanosoma; C053277:procyclic acidic repetitive protein, Trypanosoma