Structural requirements for lysosomal targeting of the prosaposin precursor protein

Authors

  • Daniela Ham

    Corresponding author
    1. Rideau Institute Advanced Research Centre for the Physical & Behavioral Sciences, 4773 Sherbrooke West, Montreal, Québec H3Z 1G5, Canada
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Corresponding author. Present address: Masaryk University, Department of Preventive Medicine, Jostova 10, 662 43 Brno, Czech Republic, Fax: +420 542 126 366. Hamdaniela@yahoo.com

Abstract

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.

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