FIG. 4.

Porin-induced Ca²⁺ influx results in increased levels of plasma membrane TfR. A431 cultures were incubated for 5 min at 37°C with Ca²⁺-containing medium alone (M) or with P1.B in Ca²⁺-replete (+) or Ca²⁺-depleted (−) medium. Certain cultures, as indicated, were exposed to P1.B preincubated with 0.1 mM ATP. (A) Plasma membrane TfR levels were determined by means of an antibody-binding assay as described in Materials and Methods. Plasma membrane TfR levels in porin-treated cultures were normalized to those for cultures exposed to medium alone. Relative TfR level in cultures treated with porin in Ca²⁺-replete medium, 1.33 ± 0.2 (P < 0.01; paired Student's t test). (B) TfR on the plasma membrane of A431 cells was detected by immunofluorescence microscopy as described in Materials and Methods.