Cell-surface receptors. Transmembrane chemosensory receptors serve to gather chemical or hormonal information at the cell surface, then trigger the appropriate intracellular pathways. In cellular chemotaxis, for example, cell surface receptors bind specific attractants and initiate cellular movement up attractant gradients. Because of their critical role in the initiation of cellular signals and their accessibility to extracellular hormones and drugs, chemoreceptors have become a prominent research target in the fields of signaling biology and medicine. The Falke group is developing an array of approaches that utilize tools of spectroscopy, structural biology, protein biochemistry, pathway reconstitution, and molecular biology to probe the structure and mechanism of prototypical chemoreceptors. A combination of these approaches has recently been successfully applied to elucidate the transmembrane conformational signal of the E. coli aspartate chemoreceptor, providing the first molecular picture of the membrane-spanning conformational signal in a receptor protein. Current goals include elucidation of the mechanisms by which receptor attractant and adaptation signals regulate a cytoplasmic kinase, thereby ultimately controlling cellular movement. A new direction of research focuses on eukaryotic G protein-coupled receptors (GPCRs) involved in the chemotaxis of mammalian immuno-defender cells, in particular neutrophils and macrophage. The same methods proven successful in the aspartate receptor are being applied to the formylated peptide receptor, a GPCR that enables neutrophils and macrophage to track down invading bacteria. Goals include the determination of receptor structure, the mechanisms of transmembrane signaling and receptor adaptation, and the molecular basis of G protein regulation. Studies of the aspartate and formylated peptide receptors have broad implications, since they typify the most prevalent classes of receptors found in prokaryotic and eukaryotic organisms, respectively.

Membrane docking motifs. Most cellular signals trigger critical regulatory events at a membrane surface. Such events modulate membrane-associated receptors, channels, peripheral membrane proteins, lipids, and other signaling molecules. Currently the Falke group is studying the molecular mechanisms of two important membrane signaling motifs that regulate responses to different second messengers: Ca(II) ions and phosphatidyl-inositol (PI) lipids. Cytoplasmic Ca(II) fluxes activate a broad array of signaling proteins, many of which contain the C2 domain, a Ca(II)-triggered membrane docking motif. Recent work in the Falke group has elucidated the Ca(II) signaling cycle of a representative C2 domain. Current work is targetting the mechanism by which Ca(II) binding drives membrane docking, and the molecular nature of the protein-membrane interaction. Furthermore, comparative studies of C2 domains from different signaling proteins are beginning to elucidate the universal and specialized features of this ubiquitous motif. Toward this end, the lab has isolated C2 domains from proteins that differ dramatically in their cellular function, including cytosolic phospholipase A2, protein kinase C, synaptotagmin, and Nedd4, for comparison. Turning to signals within the membrane itself, phosphorylated PI lipids, for example PIP3, serve as second messengers that activate signaling proteins in at specific membrane surfaces. The presence of a target PIP lipid typically drives proteins containing the pleckstrin homology (PH) domain to dock to the membrane. A new project in the Falke lab focuses on this PH motif, aiming to elucidate its mechanisms of activation and membrane docking. The lab has cloned and isolated PH domains from the signaling proteins GRP1, AKT, PLC, TAPP and DAPP, and is beginning comparative studies to determine the structure of the protein-membrane interface and its role in defining the remarkable PIP lipid specificities of individual PH domains. Together these studies of C2 and PH domains will define the molecular mechanisms of two ubiquitous membrane signaling motifs.