The receptor for C1q, gClq-R (33 kDa), is a ubiquitously expressed, multi-ligand binding, cell-surface molecule with affinity for the globular heads of Clq (gC1q). In addition to C1q, gC1q-R binds to high molecular weight kininogen (HK) as well as to diverse ligands that include, viral, bacterial microbial, cellular, and plasma proteis. While interaction of hepatitis C virus core protein with T cell gC1q-R has been shown to induce T cell suppression, recent evidence also shows that gC1q-R on CD4+ T cells is a receptor for the HIV-gp41 3S motif and plays a critical role in the autologous killing of CD4+ T cells, which is central to HIV pathogenesis. Furthermore, gC1q-R has emerged as novel receptor for pathogen associated ligands including Protein A of S. aureus, which uses platelet gC1q-R to gain access to distal organ targets of infection. Similarly, the two primary plasma ligands ! C1q and HK!use gC1q-R on a wide range of cell types to induce a plethora of biologic responses all of which contribute directly or indirectly to the inflammatory processes especially those associated with microbial infection, autoimmunity and cancer. Whereas binding of C1q to endothelial cells (ECs) leads to cellular activation followed by release of biological mediators and/or expression of adhesion molecules; the interaction of HK with gC1q-R serves as a zinc-dependent high-affinity platform for the assembly of the kinin-forming cascade. This in turn leads to the generation of bradykinin, a potent vasoactive peptide involved in various inflammatory processes including angioedema, hypotension, angiogenesis, and blood cell trafficking. During the last funding period, we dissected the structural basis of gC1q-R function using a vast array of gC1q-R deletion mutants and corresponding peptides that were engineered on the basis of prediction from the crystal to be the likely sites of interaction with the cell surface as well as with solubl ligands. Furthermore, we made an unexpected but revealing observation that gives us a new insight into how the molecule is synthesized and regulated. The present studies are therefore designed to build upon these observations. The specific aims are: 1) to identify the critical residues in gC1q-R domains that have been identified as sites of interaction with C1q and HK as well as pathogen associated ligands, 2) to solve the crystal structure of gC1q-R in complex with HK and the globular heads of C1q (gC1q), and 3) to generate gC1q-R -/- mouse for biological studies so that the role of gC1q-R in S. aureus infection and vascular permeability could be thoroughly examined and corroborated. While the previous funding period has established the foundation for the proposed studies, which are designed to address in a much greater detail the structural and mechanistic aspects of gC1q-R function, they have also armed us with a wealth of structural information that will allow us to embark on the next frontier: the translation of this knowledge into new diagnostic and therapeutic strategies that could be tested in in vivo and ex vivo disease models. Furthermore, the availability of a vast array of recombinant proteins, peptides, and antibodies will also enable us to develop sensitive screening and/or diagnostic assays to measure soluble gC1q- R whose elevated level is postulated to be a predictor of inflammatory diseases and cancer.