Published online before print January 8, 2007
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Institut für Physiologische Chemie, Philipps-Universität Marburg, Marburg, Germany
1 Correspondence: Institut für Physiologische Chemie, Karl-von-Frisch-Str. 1, 35043 Marburg/Lahn, Germany. E-mail: lemansky{at}staff.uni-marburg.de
To clarify the sorting mechanism of the lysosomal/granular proteoglycan serglycin, we treated human promonocytic U937 cells with p-nitrophenyl-ß-D-xyloside (PNP-xyl) and cycloheximide. In the absence of protein synthesis, the carbohydrate moiety of serglycin was synthesized as PNP-xyl-chondroitin sulfate (CS), and most of it was delivered to lysosomes and degraded. Further, an augmented lysosomal targeting of serglycin in the presence of tunicamycin suggested that a sorting/lectin receptor with multiple specificity was involved with an increased capacity for serglycin in the absence of N-glycosylation. Correspondingly, the cation-independent mannose 6-phosphate receptor (CI-MPR) and sortilin were observed to bind to immobilized CS. These receptors were eluted in the presence of 200–400 mM and 100–250 mM NaCl, respectively. After treating the cells with a cross-linking reagent, a portion of the sulfated proteoglycan was coimmunoprecipitated with the CI-MPR but not with sortilin. In the presence of phorbol ester, lysosomal targeting of serglycin and to a lesser extent, of cathepsin D was inhibited. We conclude that the CI-MPR participates in lysosomal and granular targeting of serglycin and basic proteins such as lysozyme associated with the proteoglycan in hematopoietic cells.
Key Words: chondroitin sulfate • lysozyme • phorbol ester • sortilin
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