Identification and characterisation of two surface glycoproteins on cultured cerebellar cells

Two high molecular weight surface glycoproteins of cerebellar cells which are selectively labeled by lactoperoxidase-catalyzed iodination of monolayer cultures from the developing mouse cerebellum, have been identified and partially characterised. Both molecules, called peak 2 and peak 3 proteins, were the major glycoprotein species detected in cerebellar cell cultures after labelling with various radioactive sugars. The freshly iodinated molecules were firmly bound to the cells, but they were released into the medium upon prolonged incubation on the cultures. The soluble peak 2 and peak 3 proteins recovered from the medium comigrated on SDS-polyacrylamide gels with their cell-bound counterparts. Thus, their release results from mechanisms other than extensive degradation. The soluble proteins eluted from gel columns corresponding to molecular weights of over 500,000 and around 300,000, for peaks 2 and 3, respectively. They bound to and were specifically eluted from concanavalin A-Sepharose columns. Peak 3 protein could be easily identified as the most prominent iodinatable polypeptide in cerebellar cell cultures. Its surface expression depended on the presence of neuronal cells. After degeneration of neuron-like cells, a component of greater molecular weight than peak 3 or 2 was predominantly labeled by surface iodination. Peak 2 protein was quantitatively precipitated from labeled culture medium by two heterologous antiseRA. Anti-peak 2 activity was removed from the antiserum by absorption with adult mouse brain, but not by liver, spleen, thymus, kidney, heart and lung. Thus, peak 2 protein may be considered as a brain-specific glycoprotein.