Compartments of tubulin and tubulin-like proteins in differentiating neubroblastoma cells.

Cytoplasmic, tubular and particulate fractions of differentiating neuroblastoma cells were prepared and the tubulin together with tubulin-like proteins was measured in each cell fraction during different stages of cell differentiation. In undifferentiated cells, 73%, 5% and 22% of the tubulin and tubulin-like proteins were contained in the cytoplasmic, tubular and particulate fractions, respectively. After 5 days of differentiation, the overall content of tubulin and tubulin-like proteins had increased by 73%. This corresponded to increases of 45%, 145% and 100% in the cytoplasmic, microtubular and particulate fractions, respectively. The increase in membrane-bound (particulate) tubulin and tubulin-like proteins was significantly greater than the total increase of proteins in the particulate fraction. Polyacrylamide gel electrophoresis of the proteins in each subcellular fraction revealed the presence of protein bands corresponding to the alpha and beta subunits of tubulin. Whereas these bands indicated equal amounts of protein in the alpha and beta positions for the tubular and particulate cell fractions, an analysis of the cytoplasmic fraction revealed much more protein migrating to the alpha-tubulin position than to the beta-tubulin position, especially during cell differentiation. Furthermore, two overlapping but distinct protein bands were demonstrable in the position of the alpha-tubulin from the cytoplasmic fraction. These bands were designated alpha 1 and alpha 2. The particulate fraction contained only the alpha 1 and the tubular fraction only the alpha 2 protein band. The addition of 1 mM dibutyryl cyclic AMP to the neuroblastoma cells, at the time when the serum was withdrawn, enhanced the rate of differentiation and the redistribution of tubulin and tubulin-like proteins within the 3 cellular compartments. These results are discussed as they relate to the regulation, biosynthesis, turnover and compartmentation of tubulin and tubulin-like proteins in differentiating neuroblastoma cells.