A MAJOR BINDING-PROTEIN FOR LEUKEMIA INHIBITORY FACTOR IN NORMAL MOUSE SERUM - IDENTIFICATION AS A SOLUBLE FORM OF THE CELLULAR RECEPTOR

A protein that specifically binds leukemia inhibitory factor (LIF) has been isolated from normal mouse serum by using four successive fractionation steps: chromatography on a LIF affinity matrix, anion-exchange chromatography, size-exclusion chromatography, and preparative native gel electrophoresis. The purified LIF-binding protein (LBP) is a glycoprotein with an apparent molecular mass of 90 kDa that specifically binds I-125-labeled murine LIF with an affinity comparable to that of the low-affinity cellular LIF receptor (K(d) = 600 pM). N-terminal sequencing has identified this protein as a soluble truncated form of the a chain of the cellular LIF receptor. LBP is present in normal mouse serum at high levels (1-mu-g/ml) and these levels are elevated in pregnant mice and reduced in neonatal mice. Since normal serum concentrations of LBP can block the biological actions of LIF in culture, LBP may serve as an inhibitor of the systemic effects of locally produced LIF.