Phosphorylation of neurofilaments and microtubule-associated proteins (MAPs) are topologically regulated within neurons. For example, neurofilaments (NFs) (NF-M and NF-H) and tau proteins are extensively phosphorylated in the axon but not in the cell bodies. However, in certain neurodegenerative diseases these proteins are extensively phosphorylated in the cell bodies. The mechanisms of this phosphorylation is not known. Cytoskeletal protein phosphorylation is a complex interplay among kinases, phosphatases and their regulators. To understand this topographic regulation of cytoskeletal protein phosphorylation, we used squid as a model system where giant axons can be isolated free from their perikarya in the giant fiber lobe (GFL) of the stellate ganglion. The phosphorylation mechanisms in these distinct cellular domains can be studied and compared biochemically. Since P13suc1 binds with high affinity to cdc2-like kinases, we used P13suc1 affinity chromatography, coupled to immunoblot analysis, to study proteins associated with the P13suc1/cdc2-like kinase complex isolated from axoplasm and the GFL. Although lysates from axoplasm and the GFL both contained qualitatively similar protein profiles as determined by immunoblot analyses, it was only from axoplasm that we extracted an active P13 multimeric complex of cytoskeletal proteins, (tubulin, NFs, actin and kinesin), their phosphorylating kinases and regulators. No such active P13 complex was obtained from perikarya of the GFL; the GFL/P13 complex lacked tubulin, NFs and most kinases, although a cdc2- like kinase was detected, and exhibited low levels of phosphorylating activity. We propose that kinases, cytoskeletal proteins and regulators, though synthesized in the neuronal cell body, assemble in the axon as a multimeric complex which actively regulates phosphorylation of elements of the axonal cytoskeleton. Currently, we are analyzing these complexes in more detail and are also studying the mechanisms underlying the formation of these multimeric complexes in the axonal compartments of the neuron.
