Fig 1: LDL triggers the endocytosis of ALK-1.A, HUVECs were incubated with LDL (25 µg/ml) for the indicated times and biotinylated cell surface proteins isolated for immunoblotting. B–E, levels of cell surface LDLR, ALK-1, and SR-B1 after treatments of HUVEC with LDL (B, 25 µg/ml) or BMP-9 (D, 0.5 ng/ml) for the indicated time (0–30 min) and quantified in C and E. F and G, HUVEC were transfected with siRNAs for 48 and incubated with LDL (25 µg/ml) for 60 min; ALK-1 on the cell surface detected using biotin-labeling and the quantified G. Quantitative data were from three independent experiments.