CHARACTERIZATION OF A BINDING-PROTEIN FOR LEUKEMIA INHIBITORY FACTOR LOCALIZED IN EXTRACELLULAR-MATRIX

Leukemia Inhibitory Factor (LIF) interacts with two classes of high af finity binding sites on rat UMR cells cultured in monolayer. One class of binding sites was found to be localized in the extracellular matri x (ECM) after removal of cells from the culture dish. The interaction of LIF with ECM-localized binding sites is not dependent upon either g lycosylation of LIF or the presence of extracellular glycosyaminoglyca ns. Chemical cross-linking studies demonstrate that LIF interacts with a 200-kD cell-associated protein and a 140-kD ECM-localized protein. A 140-kD protein could also be specifically precipitated from solubili sed metabolically radiolabeled UMR ECM by antibodies directed against LIF by virtue of its ability to form a stable complex with unlabeled L IF. In addition, soluble LIF associated with this ECM-localized protei n is biologically active in terms of inhibition of ES cell differentia tion. The properties of ECM-localized 140-kD species are very similar to those of the secreted form of the LIF receptor suggesting that the ECM localization of LIF and LIF signal transduction may be closely cou pled.