This project combines resources of two established laboratories in membrane protein biophysics and biochemistry to study details of the intermolecular interactions of the epidermal growth factor (EGF) receptor with its ligands and with other receptors of the ErbB family of receptor kinases. The ErbB family of receptors, and, in particular, the EGF receptor, are among the most widely studied polypeptide hormone receptors because of their importance in normal development, wound healing, and, when deregulated by mutation or over-expression, neoplasia. Nonetheless, fundamental questions concerning the mechanisms of signal transduction triggered by the binding of the EGF family of ligands to the ErbB family of receptors remain unresolved. The long term goal of this project is to develop an in-depth understanding of the earliest events in the signal transduction pathway that occur when EGF family ligands bind to their respective receptors. The specific aims for this budget period will exploit well characterized fluorescent ligands, new cell lines that have been shown to withstand turbulent stoppedflow rapid mixing and that express selected ErbB family receptors in the absence of endogenous ErbB receptors, as well as a custom stopped-flow fluorometer specifically designed and optimized for these studies. Studies will address hypotheses a) that expression of other ErbB family members affects the proportion of EGF receptors in high and low affinity states, b) that ligands recognized by ErbB3 or ErbB4 can modulate the binding of EGF to its receptor when the receptors are present in the same cell, c) that the dynamic on and off rates for receptor binding are important parameters for determining the mitogenic activity ofpeptide hormones, and d) that heterogeniety in glycosylation can affect receptor affinity for EGF. Studies will also test competing hypotheses concerning the interactions between the EGF receptor and ErbB2: that EGF receptor-ErbB2 dimers form directly when EGF binds to the receptor, or that EGF receptors dimerize on binding EGF and subsequently recruit ErbB2.