Analysis of the influences of the E5 transforming protein on kinetic parameters of epidermal growth factor binding and metabolism

The E5 protein of the bovine papillomavirus induces cellular transformation when transfected into NIH 3T3 cells, and the extent of focal transformation is enhanced by cotransfection with the epidermal growth factor (EGF) receptor (Martin et al., Cell 59:21–32, 1989). To determine whether E5 affects EGF:receptor interactions, we analyzed the kinetics of ¹²⁵I‐EGF processing using a mathematical model that enabled us to evaluate rate constants for ligand association (k_a), dissociation (k_d), internalization (k_e), recycling (k_r), and degradation (k_h). These rate constants were measured in NIH 3T3 cells transfected with the human EGF receptor (ER cells) and in cells transfected with both the EGF receptor and E5 (E5/ER cells). We found that the rate constant for ¹²⁵I‐EGF association k_a was significantly decreased in E5/ER cells, but was apparently occupancy‐independent in both cell lines. The ¹²⁵I‐EGF dissociation rate constant k_d was significantly lower in E5 transformed cells, and increased with occupancy in both cell lines. This suggests that E5 alters the receptor before or during EGF binding so that ligand association is slower; however, once complexes are formed, EGF is bound more tightly to the receptor. Rate constants for internalization k_e were also found to be occupancy‐dependent, although at a given level of occupancy k_e was similar for both cell lines. Also, there was no apparent effect of E5 on the recycling rate constant k_r. The ¹²⁵I‐EGF degradation rate constant k_h was 30% lower in E5 transformed cells, and was occupancy‐independent. The overall effect of E5 is to stabilize intact EGF:receptor complexes which may alter mitogenic signaling of the receptor. © 1992 Wiley‐Liss, Inc.