The goal of my proposed research is to characterize lipid-mediated lateral organization in cell plasma membranes and to decipher the underlying physical mechanisms that give rise to this structure. In so doing, I will surmount significant barriers currently facing cell membrane research by developing new experimental tools to manipulate and test the functional consequences of lipid-mediated organization in living cells. My past success in deciphering liquid immiscibility in simple model membranes forms the foundation for my current and future postdoctoral studies in biological membranes. Since joining the Baird Laboratory at Cornell University, I have identified robust critical fluctuations in plasma membrane vesicles isolated from RBL mast cells. This result strongly supports my working hypothesis that critical fluctuations are present in cell plasma membranes at physiological temperatures. In the mentored stage of this award, I will begin to directly test my working hypothesis by implementing experimental and analytic techniques to quantify and interpret nanometer-size organization on the intact cell surface (Aim1). 1 will also develop experimental methods to modulate lateral heterogeneity in both plasma membrane vesicles and intact cell plasma membranes (Aim 2). Through these studies, I will acquire the skills and perspective necessary to successfully apply my strong physical background to this biomedically relevant area of research. As an independent researcher, I will continue work on both aims, eventually deciphering the physical basis of membrane lateral organization (Aim1), and developing novel methodologies to probe the functional consequences of lipid-mediated lateral heterogeneity in IgE mediated signaling in RBL mast cells (Aim 2). My long term goal is develop general methods to probe the functional roles of lipids in a wide range of processes and cell types which will enable new lines of study into the biomedical consequences of lipid mediated plasma membrane organization in cells. The Pathway to Independence Award will give me the resources to focus deeply on my biological training in my postdoctoral years, and the independence to pursue my own research objectives as I transition into my future position as a young faculty member. Relevance: It is widely accepted that plasma membrane lateral heterogeniety plays important functional roles in normal cell processes and in the development and maintenance of human diseases. I will open new opportunities for intervention in cell membrane-based pathologies by developing effective experimental techniques and a clear conceptual framework in which to study lipids and proteins in native membranes.