title: The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
abstract: Most soluble lysosomal proteins bind the mannose 6-phosphate receptor (M6P-R) to be sorted to the lysosomes. However, the lysosomes of I-cell disease (ICD) patients, a condition resulting from a mutation in the phosphotransferase that adds mannose 6-phosphate to hydrolases, have near normal levels of several lysosomal proteins, including the sphingolipid activator proteins (SAPs), GM2AP and prosaposin. We tested the hypothesis that SAPs are targeted to the lysosomal compartment via the sortilin receptor. To test this hypothesis, a dominant-negative construct of sortilin and a sortilin small interfering RNA (siRNA) were introduced into COS-7 cells. Our results showed that both the truncated sortilin and the sortilin siRNA block the traffic of GM2AP and prosaposin to the lysosomal compartment. This observation was confirmed by a co-immunoprecipitation, which demonstrated that GM2AP and prosaposin are interactive partners of sortilin. Furthermore, a dominant-negative mutant GGA prevented the trafficking of prosaposin and GM2AP to lysosomes. In conclusion, our results show that the trafficking of SAPs is dependent on sortilin, demonstrating a novel lysosomal trafficking.
authors.affiliation: Department of Anatomy and Cell Biology, McGill University, 3640 University Montreal, Quebec, Canada H3A 2B2.
authors.forename: Stephane
authors.initials: S
authors.lastname: Lefrancois
authors.forename: Jibin
authors.initials: J
authors.lastname: Zeng
authors.forename: A Jacob
authors.initials: AJ
authors.lastname: Hassan
authors.forename: Maryssa
authors.initials: M
authors.lastname: Canuel
authors.forename: Carlos R
authors.initials: CR
authors.lastname: Morales
mesh_terms: D033942:Adaptor Proteins, Vesicular Transport; D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008247:Lysosomes / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D016296:Mutagenesis; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics; D017527:Receptor, IGF Type 2 / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D049231:Saposins; D017384:Sequence Deletion; D049128:Sphingolipid Activator Proteins; D000099183:Sortilin
chemical_list: D033942:Adaptor Proteins, Vesicular Transport; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D034741:RNA, Small Interfering; D017527:Receptor, IGF Type 2; D011993:Recombinant Fusion Proteins; D049231:Saposins; D049128:Sphingolipid Activator Proteins; D000099183:Sortilin; C483345:PSAP protein, human