Polypeptides similar to the α and β subunits of tubulin are exposed on the neuronal surface

Abstract

THE finding by Borisy and Taylor¹ that nervous tissue was the source of large amounts of soluble tubulin, the subunit protein of microtubules, has greatly facilitated the biochemical study of microtubular structure. From 10% to 20% of the total protein of developing brain is in the form of soluble tubulin^2,3 and most studies have focused on this form of the protein. In 1970, however, Feit and Barondes demonstrated a large amount of colchicine binding activity, a term which has become associated with the presence of tubulin, in particulate fractions of brain⁴—fractions expected to be free of microtubule-derived tubulin. Recent studies indicate that polypeptides with the same size and drug-binding properties as soluble tubulin are present in synaptosomal plasma membranes^5–7. These studies suggest a membrane-associated form of the protein which is insoluble in non-chaotropic buffers. I here report experiments which provide additional evidence for membrane-associated tubulin-like proteins and also suggest that a portion of this polypeptide(s) may be exposed on the external surface of the cell.