Fig. 1. (A) Downregulation of PDGF ß-receptor by LT on ST15A cells, containing a temperature-sensitive mutant of LT cultured at 33°C or 39°C. The amount of PDGF receptor binding was analysed by addition of serial dilutions of cold PDGF ligand to compete with [¹²⁵I]PDGF ligand for cell binding. The -receptor was depleted by preincubation of cells with PDGF-AA at 37°C. (B) Photomicrographs of ST15A cells immunostained with a PDGF ß-receptor antibody after culture at 33°C (left) and after differentiation at 39°C (right).