Figure 1.

Internalization of aFGF in cells transfected with FGFR4. (A) COS cells transiently transfected with FGFR4 were incubated at 37°C for the indicated periods of time in HEPES containing 10 U/ml heparin and 50 ng/ml ¹²⁵I-aFGF. Then the cells were washed with PBS and treated for 6 min with 20 mM Na-acetate, pH 4, containing 2 M NaCl to remove surface-bound aFGF. The removed material was collected. The cells were subsequently dissolved, and the radioactivity in the medium and in the cells was measured. The data are expressed as the internalized radioactivity as a percentage of the total radioactivity associated with the cells. (B) Uptake of ¹²⁵I-transferrin (▴) and ¹²⁵I-aFGF (●) by COS cells transfected with FGFR4. The cells were incubated for 5 min at 37°C in HEPES medium, pH 5.5, with increasing concentrations of acetic acid. ¹²⁵I-Transferrin or ¹²⁵I-aFGF was then added as described above, and after 5 and 15 min of incubation, respectively, the amount of endocytosed proteins was measured as in A.