Interaction of glia maturation factor with the glial cell membrane.

Glia maturation factor (GMF) immobilized on agarose beads retained the same mitogenic and morphological transforming activities as free GMF when tested on glioblasts. The exposure of glioblasts to immobilized GMF for 5 min provoked the initiation of DNA synthesis and maximal stimulation was obtained within 30 min. Thiol-reducing agents, such as dithiothreitol and cysteine, increased the biological activity of GMF. These data suggest the presence of a surface receptor to GMF on the glioblasts, while the reduction of the thiol group(s) in GMF promotes the binding to its receptor. Repeated use of immobilized GMF decreased both the mitogenic and the morphological transforming activities. Immobilized GMF used for the third time lost its biological activity. This implies the existence of a certain kind of degradation system such as a proteolytic enzyme located close to the GMF receptor on the glial cell surface.