The BPV‐1 E5 protein, the 16 kDa membrane pore‐forming protein and the PDGF receptor exist in a complex that is dependent on hydrophobic transmembrane interactions

The E5 oncoprotein of bovine papillomavirus type 1 is a 44 amino acid, highly hydrophobic protein that induces the stable transformation of immortalized murine fibroblasts, presumably through its activation of growth factor receptors. Previous studies have shown that the E5 protein complexes with the 16 kDa (16k) pore‐forming protein of vacuolar H(+)‐ATPases. This integral membrane protein is essential for the acidification and function of subcellular compartments that process growth factor receptors. Using an SV40 expression system in COS cells, we analyzed whether the E5‐16k complexes bind additional cellular proteins, including growth factor receptors. These studies demonstrate that E5 binds to both the 16k protein and the PDGF receptor and that this tri‐component complex can be isolated with antibodies specific for each protein. Importantly, the 16k protein bound to the PDGF receptor in the absence of E5, suggesting that E5 binds to the PDGF receptor via its interaction with the 16k protein. An E5 mutant lacking the hydrophilic carboxyl‐terminal 14 amino acids retained binding to both 16k and the PDGF receptor, indicating that E5 binds to these proteins through its hydrophobic, membrane‐associating domain. These studies reveal that hydrophobic, intramembrane interactions govern the association of E5, 16k and the PDGF receptor, suggesting a ligand‐independent mechanism for receptor activation and a potential link between receptor signal transduction pathways and membrane pore activity.