Expression of External‐Surface Membrane Proteins in Differentiated and Undifferentiated Mouse Neuroblastoma Cells

Murine neuroblastoma cultures were labeled externally with the cationic reagent N,N,N‐[³H] ‐trimethylamino‐β‐alalyl‐N‐hydroxy‐succinimide ester ([³H]Me₃N‐βAla‐NSuc) or with ¹²⁵I/lactoperoxidase. The cells were labeled in the logarithmic and confluent growth phases as well as in a highly differentiaed state following treatment with 2% dimethylsulfoxide. The labeled exterior membrane proteins were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Major changes in the exterior membrane proteins were observed during maturation and differentiation of the cells. Most of these changes were clonal‐specific, while others were common to several clones. Two proteins of M_r 55000 and 65000 were labeled by both ¹²⁵I/lactoperoxidase and Me₃N‐[³H]‐βAla‐NSuc. The level of labeling was dependent on the clonal lines used and the state of the cell maturation. A group of proteins displaying a molecular weight between 150000 and 200000 was found to be related to the transition of a culture from logarithmic to confluent growth phases. An additional protein, with an apparent molecular weight of 95000, was common to differentiated cells of the two inducible clones used. In general the maturation of logarithmic phase cells into confluent cells resulted in a less complex electrophoretic distribution of the pattern of labeling. After dimethylsulfoxide treatment, further reduction in the complexity of the externally labeled proteins was observed.