Fig. 5. PDGF and fibronectin promote endocytosis of cell-surface tTG. (A) PDGF accelerates internalization and degradation of cell-surface tTG. Quiescent serum-starved WI-38 fibroblasts surface-labeled with sulfo-NHS-SS-biotin, were either left untreated [`C'], or were treated with (c) PDGF only, or with (h) PDGF and the inhibitors of clathrin-dependent (0.45 M sucrose) or (f) caveolae-dependent (5 µg/ml filipin) endocytosis. (B) The internalization of tTG from the cell surface is stimulated by fibronectin. Fibronectin-null mouse embryonic fibroblasts expressing exogenous tTG were grown in fibronectin-depleted 10% serum with or without 50 µg/ml fibronectin (Fn) and were surface-biotinylated using sulfo-NHS-SS-biotin. Biotinylated proteins were isolated and tTG was detected by SDS-PAGE and immunoblotting in the fraction of proteins internalized from the cell surface (A,B). Proteolytic fragments of tTG are marked with asterisks. Shown in (A,B) are representative results of three independent experiments.