The growth cone cytoskeleton. Glycoprotein association, calmodulin binding, and tyrosine/serine phosphorylation of tubulin

Cytoskeletons were prepared from the growth cones of neonatal rat forebrains and were utilized to explore several aspects of growth cone function. The cytoskeletal fraction retained about 50% of total growth cone protein, was highly enriched in tubulin, and constituted an interconnected lattice of 10-25 nm homogeneous particles. The cytoskeleton appeared to be a target for Ca2+ signaling since it contained the majority of growth cone calmodulin-binding polypeptides which featured prominently an Mr 135,000 component. Most of the growth cone glycoproteins were at least partially associated with the cytoskeleton, thus suggesting the possibility of a transmembrane coupling mechanism which allows for communication between the cytoskeleton and the external surface of the growth cone. The cytoskeleton was also endowed with one or more protein kinases which phosphorylated endogenous and exogenous tubulin achieving a stoichiometry of 9-13 mol of phosphate/mol of substrate dimer. Interestingly, the site of tubulin phosphorylation included tyrosine as well as serine residues providing a possible target for the action of neuronal tyrosine kinases such as growth factor receptors or oncogene products. Finally, a comparison between cytoskeletal preparations from growth cones and from mature synaptosomes revealed several differences in glycoprotein association, calmodulin binding, and protein phosphorylation, evidently reflecting maturational events which might underlie relevant aspects of synaptogenesis.