Antiserum against a rat neuronal tumor cell line (B103) has been prepared in rabbit by intravenous injection of live cells. This immune serum (anti-B103) precipitates a few cell surface proteins recognizable by two-dimensional gel electrophoresis as common radioiodinatable spots in 15 different rat neural cell lines and in mouse and rat fibroblast cell lines. The apparent molecular weight of one major common protein (II4) is estimated by SDS gel electrophoresis to be somewhere between 80,000 and 90,000 and another protein (I3) to be 120,000. These two proteins are consistently recognized in various cell lines by this antiserum. Furthermore, at a 1:20 dilution, this serum causes monolayer cells to round up usually within 0.5 h and detach from the plate within 3 h. It also inhibits spreading of freshly plated cells. These effects of the antiserum are reversible. Upon absorption of the antiserum with cells (e.g., absorbed with a glial cell line, B27), the serum no longer causes the rounding up of the monolayer cells, it does not inhibit cell spreading, and it does not immune-precipitate the two common proteins from the cell surface of various cell lines. Antisera against several other rat cell lines also precipitate the same common proteins (II4 and I3) from the cell surface and prevent cell spreading. These data suggest that the antibody acts first at the cell surface and then inhibits cell spreading or rounding up of spread cells. The consistent pattern of the immunoprecipitated cell surface proteins on the two-dimensional gel electrophoresis makes these two common surface proteins (II4 or I3 or both) possible candidates for target proteins to which the antibody binds. Thus, they may play a critical role in cell spreading.

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