Glycobiology vol 7 no 1 pp. 103-112, 1997
© 1997
research-article |
Glypican (heparan sulfate proteoglycan) is palmitoylated, deglycanated and reglycanated during recycling in skin fibroblasts
Department of Cell and Molecular Biology, Faculty of Medicine, Lund University Lund, Sweden
1To whom correspondence should be addressed at: Department of Cell and Molecular Biology 1, POB 94, S-221 00, Lund, Sweden
Received on June 12, 1996; revised on August 22, 1996; accepted on August 26, 1996
Skin fibroblasts treated with brefeldin A produce a recycling variant of glypican (a glycosyiphosphatidylinositol-anchored heparan-sulfate proteoglycan) that is resistant to inositol-specific phospholipase C and incorporates sulfate and glucosamine into heparan sulfate chains (Fransson, L.-Å. et al., Glycobiology, 5, 407415, 1995). We have now investigated structural modifications of recycling glypican, such as fatty acylation from [3H]palmitate, and degradation and assembly of heparan sulfate side chains. Most of the 3H-radioactivity was recovered as lipid-like material after de-esterification. To distinguish between formation of heparan sulfate at vacant sites, elongation of existing chains or degradation followed by re-elongation of chain remnants, cells were pulse-labeled with [3H]glucosamine and then chase-labeled with [14C]glucosamine. Material isolated from the cells during the chase consisted of proteoglycan and mostly [3H]-labeled heparan-sulfate degradation products (molecular mass, 2080 kDa) showing that the side chains were degraded during recycling. The degradation products were initially glucuronate-rich, but became more iduronate-rich with time. The glypican proteoglycan formed during the chase was degraded either with alkali to release intact side chains or with heparinase to generate distally located chain fragments that were separated from the core protein, containing the proximally located, covalently attached chain remnants. All of the [14C]-radioactivity incorporated during the pulse was found in peripheral chain fragments, and the chains formed were not significantly longer than the original ones. We therefore conclude that newly made heparan-sulfate chains were neither made on vacant sites, nor by extension of existing chains but rather by re-elongation of degraded chain remnants. The remodeled chains made during recycling appeared to be more extensively modified than the original ones.
fatty acylation glypican heparan sulfate recycling reglycanation
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