PROJECT SUMMARY/ABSTRACT The challenge addressed by this proposal is to generate a map, the global human cell-surface interactome, that defines in vitro interactions among the extracellular domains of human cellsurface proteins (CSPs) and secreted proteins. This map will have a major impact on biomedical research, because cell-cell interactions mediated by CSPs are central to human physiology, controlling almost every biological process that is affected by disease. CSPs and secreted ligands comprise the majority of the therapeutic targets that have been successfully developed in recent years. Knowledge of interaction partners is essential for assessing the therapeutic potential of a CSP, since this knowledge defines the biological processes that it controls. For example, PD-1 was identified as a negative regulator of T cell function in 1992, but its value as a target for cancer immunotherapy only became clear much later, when its ligand PD-L1 was identified and found to be expressed on tumor cells. We will not only generate a complete map of in vitro interactions among human CSPs and secreted proteins, but also assess the functions of these interactions in cells of the human immune and nervous systems. This is a huge project, because there are about 2000 human single-transmembrane domain CSPs and 200 “orphan” secreted factors. Creation of a map of pairwise interactions among all of these proteins requires testing 4.8 million interactions. This is beyond the capacity of current screening methods, so execution of this screen at an academic institution will require the development of new technologies. This project is too large to be supported by a traditional RO1, but is perfectly suited to the transformative research award mechanism. Here we propose new ways to multiplex both in vitro biochemical screens and in vivo functional screens, so as to make it possible to define all in vitro interactions among CSPs and secreted ligands and to assess the functions of many of these within a 5-year funding period. To do this, we will first multiplex and sensitize in vitro interactome screens using color-coded beads and high-avidity nanoparticles. We will then develop methods to convert in vitro protein interaction screens into high-throughput DNA sequencing screens, which have a huge multiplexing capacity. For the functional screens, multiplexing single- cell analysis of cell fate perturbations can allow us to assess the effects of many different ligands on single immune system and neural cells in a single experiment. The rationale for the overall approach described here is that it defines a stepwise process in which we systematically develop and optimize screen technologies, then use the technology that performs best for execution of the actual screens.