Lysophosphatidic acid (LPA) is the structurally simplest phospholipid in nature, the building block of complex phospholipids. It is a physiologic constituent of blood plasma and other biologic fluids; its increased concentration is indicative for inflammation and cancer. It is a survival factor, activates migration and differentiation of cells acting on cognate G protein-coupled receptors and the transcription factor PPAR. The activity of cell-surface receptors can lead to the generation of LPA in the intracellular layer of the plasma membrane under physiologic or pathologic conditions, which elicits, because of its conical shape, membrane-curvature stress involved in forming membrane vesicles and tubules, as well as in cell division. We hypothesized that the temporary LPA-enriched sites in the inner leaflet can serve as specific binding sites for several domains of signaling proteins, due to the membrane-packing defect and curvature stress. The appearance of such dynamic binding sites within the signaling processes might modify the actual interaction-pattern of the signaling protein network.

We have shown recently that the SH3 domain of human Caskin1 scaffold protein effectively binds to LPA-enriched surfaces instead of polyproline-containing sequences (Cellular Signaling 32, 66-75, 2017). We recognized in preliminary experiments that LPA is capable for binding to several PH and SH2 domains as well. In this project, we investigate in detail the LPA-binding of the SH2 domain of human Nck1 adaptor protein, as well as of the PH domain of human Grp1, an activator protein for the regulator Arf1. Our preliminary data revealed the selectivity of LPA binding as neither domains bind lipid mediators structurally or functionally linked to LPA. We will study the affinity and stoichiometry of the domains' binding to LPA-micelles and LPA-containing liposomes as membrane models (Károly Liliom, ITC, QCM, MST), and aim to identify the structural elements involved in binding on both domains (Judith Mihály, ATR-FTIR), whose 3D-structures have been published. We also will seek for potential inhibition by LPA binding of the known interactions of the domains, a phosphotyrosine-peptide binding in case of the SH2, while a phosphatidylinositol polyphosphate binding for the PH domain. Such inhibition would raise exciting new possibilities in the tuning of signal transduction.

Judith Mihály - Károly Liliom

Report I. - 2017 June