Structural requirements for lysosomal targeting of the prosaposin precursor protein.
Although the Man-6-P-independent lysosomal sorting of prosaposin, a precursor of four saposins (A, B, C, and D) is not understood, a protein/lipid interaction is considered. Immunocytochemical analysis revealed that each single saposin linked to the C-terminus of prosaposin and to secretory albumin, drives the chimeric protein to lysosomes in COS-7 cells. Quantitative image analysis demonstrated that saposins are targeted with different efficiency (P<0.05) and in a less smooth manner than the precursor. Despite a very close homology, the charge distribution at the surface of 3D comparative models between saposins appeared different. Western blotting monitored prosaposin in cells also as a di- or trimeric form, whereas the chimeric saposins as monomeric. This implies that each amphipathic saposin-like motif may be a part of the overall structural requirements for binding of the precursor to the membrane lipids of transport vesicle. The crystal structure of saposin B demonstrating two dimeric units for lipid binding supports current findings.
Ham
Daniela
D
Rideau Institute Advanced Research Centre for the Physical & Behavioral Sciences, 4773 Sherbrooke West, Montreal, H3Z 1G5, Québec, Canada. Hamdaniela@yahoo.com
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D017124:Conserved Sequence; D006023:Glycoproteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007150:Immunohistochemistry; D008247:Lysosomes / Q000378:metabolism*; D008297:Male; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D008969:Molecular Sequence Data; D011498:Protein Precursors / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D049231:Saposins; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D006023:Glycoproteins; C483345:PSAP protein, human; D011498:Protein Precursors; C483347:Psap protein, rat; D049231:Saposins