BIOCHEMICAL-CHARACTERIZATION AND IMMUNOLOCALIZATION OF SC2 PROTEIN - SC2 PROTEIN IS INDISTINGUISHABLE FROM THE CELL-ADHESION MOLECULE AXONIN-1

SC2 is a monoclonal antibody that was previously shown to recognize a subset of neurons in the developing nervous system of the chick. We ha ve now used the SC2 monoclonal antibody to purify from chick embryo br ain membranes a glycoprotein that migrates at similar to 125 kDa on SD S/PAGE. The size of this protein and its distribution pattern in the s pinal cord are similar to that observed for axonin-1. A polyclonal ant i-axonin-1 antibody R26 specifically reacted with the SC2 protein from brain. This antibody, as well as a polyclonal antibody (369) against purified SC2 protein, reacted with 115-130 kDa proteins in vitreous hu mor, a rich source of axonin-1, and with similar sized proteins precip itated from vitreous humor by the 369, and SC2 antibodies. Treatment o f SC2 protein isolated from chick brain membranes with PI-PLC indicate d that it contains a glycophosphatidylinositol (GPI) moiety. Co-aggreg ation experiments using Covaspheres with covalently bound proteins ind icated that SC2 protein binds heterophilically to Ng-CAM. Immunohistoc hemical analysis of chick embryos showed that SC2 protein is abundant in the sensory nerve bundles of both the central and peripheral nervou s systems during development. Its expression was restricted and it was specifically localized in the dorsal funiculus of the spinal cord, as well as in olfactory, retinal, trigeminal, vestibulocochlear, glossop haryngeal and vagal nerve fibers. The biochemical and immunohistochemi cal data show that SC2 protein is axonin-1, and the immunolocalization studies support the hypothesis that SC2 protein may play a role durin g development of particular fiber systems by interacting with other ce ll adhesion molecules such as Ng-CAM.