Figure 4.

Comparison of internalization rates of wild-type TR and various TR TM mutants. Equivalent numbers of CEFs expressing wild-type TR and TR_TM Thr¹¹Gln¹⁷Thr¹⁹ (A) or Ii_TM 11–19 and Ii_TM 11–19 Ala¹¹Ala¹⁷Ala¹⁹ chimeras (B) were incubated with prewarmed (37°C) ¹²⁵I-labeled Tf for indicated times. The amounts of internalized (internal Tf) and surface associated (surface Tf) radiolabel were determined as described under MATERIALS AND METHODS. Data are plotted with IN/SUR method, in which the slope of the line equals the endocytic rate constant k_e (Wiley and Cunningham, 1982). The data represent the mean ± SE from five experiments for each time point.