Tyrosine kinases are important regulators of growth, differentiation, and apoptosis in eukaryotic cells. Inappropriate tyrosine kinase signaling is observed in many cancers and inflammatory diseases. This project focuses on the structure, activity, and regulation of the Ack1 and Brk nonreceptor tyrosine kinases. In collaboration with Dr. Frederic Geissmann and colleagues, we have discovered a series of loss-of- function mutations in Ack1 and Brk in a cohort of patients with severe Systemic Lupus Erythematosus (SLE). These are the first tyrosine kinase mutations found to be linked to this disease. Our preliminary work shows that the mutations drastically decrease (or completely block) kinase activity, downstream signaling, and phagocytosis of apoptotic cells. We propose to conduct functional studies of the wild-type and mutant forms of Ack1 and Brk in mammalian cells, including experiments in induced pluripotent stem cell (iPSC)- derived macrophages from the lupus patients and their healthy relatives. We will carry out global screens of substrate specificity to determine whether the mutations “rewire” cellular signaling networks. We will also carry out mechanistic and structural studies on the purified mutant kinases, and test how they are coupled to the cell surface receptor MerTK. The overall hypothesis is that Ack1 and Brk link recognition of phosphatidylserine on apoptotic cells to phagocytic engulfment. Regulation or tuning of this signaling pathway could provide a new strategy for therapeutic approaches in autoimmune diseases.