The effects of post‐translational processing on dystroglycan synthesis and trafficking¹

Abstract

Dystroglycan is a component of the dystrophin glycoprotein complex that is cleaved into two polypeptides by an unidenti¢ed protease. To determine the role of post-translational processing on dystroglycan synthesis and tra⁄cking we expressed the dystroglycan precursor and mutants thereof in a heterologous system. A point mutant in the processing site, S655A, prevented proteolytic cleavage but had no e¡ect upon the surface localisation of dystroglycan. Mutation of two N-linked glycosylation sites that £ank the cleavage site inhibited proteolytic processing of the precursor. Furthermore, chemical inhibition of N-and O-linked glycosylation interfered with the processing of the precursor and reduced the levels of dystroglycan at the cell surface. Dystroglycan processing was also inhibited by the proteasome inhibitor lactacystin. N-linked glycosylation is a prerequisite for e⁄cient proteolytic processing and cleavage and glycosylation are dispensable for cell surface targeting of dystroglycan. ß

Expression of dystroglycan correlates with tumor grade and predicts survival in renal cell carcinoma

“…On the other hand, as previously mentioned, the a-subunit of the DG complex mediates its interaction with ECM components, is essential for a functional complex and its lack of detection indicates the loss of a functional DG complex. 7,8,13,15 In normal renal parenchyma a positive membranous staining for a-DG was always observed in the glomeruli while staining was mainly cytoplasmic in tubular cells (Fig. 1A).…”

“…7 It is formed by two subunits which are encoded by a single gene and are formed as one precursor protein cleaved into two mature proteins which form a tight non-covalent complex. 8 The a (extracellular) and b (transmembrane) subunits bind to the major ECM components and proteins involved in signal transduction and cytoskeleton organization, respectively. Thus, DG connects the ECM network to the cytoskeleton and is likely involved in the regulation of signaling pathways.…”

Filamin A Binding Stabilizes Nascent Glycoprotein Ibα Trafficking and Thereby Enhances Its Surface Expression

“…Finally, we have provided additional evidence that some GpIb␣ not bound to filamin is directed to the cell surface but that GpIb-IX without its cytoskeletal connections may be more susceptible to extracellular proteolysis. Although novel, these results are not entirely unexpected, as there are data that other proteins are chaperoned toward productive biosynthesis by filamin binding (13)(14)(15), other unprocessed glycoproteins undergo ER-directed proteasomal degradation (21,22), and uncomplexed GpIb␣ expressed on the cell surface is particularly vulnerable to extracellular proteases (8). Nonetheless, these results are useful not only because they elucidate mechanisms underlying the cell biology of megakaryocytes or the pathophysiology of Bernard-Soulier syndrome, but because they may someday facilitate organizing and optimizing an approach to cell engineering or gene therapy aimed at manipulating or restoring platelet function for therapeutic purposes (23,24).…”

Dystroglycan expression is reduced during prostate tumorigenesis and is regulated by androgens in prostate cancer cells

“…Since DG subunits are encoded by a single gene and are formed upon cleavage of a precursor protein (Henry and Campbell, 1999;Holt et al, 2000;Esapa et al, 2003), detection of the b-DG subunit in the cell lines and in most of the cancers in which a-DG was not detectable (Figs. 1 and 2) suggests that, as previously reported in other types of human malignancies, this lack of detection is likely not due to loss of gene expression but to a specific posttranscriptional mechanism affecting a-DG processing in prostate cancer cells.…”