Topographic regulation of cytoskeletal protein phosphorylation by multimeric complexes in the squid giant fiber system

In mammalian and squid nervous systems, the phosphorylation of neurofilamen t proteins (NFs) seems to be topographically regulated. Although NFs and re levant kinases are synthesized in cell bodies, phosphorylation of NFs, part icularly in the lys-ser-pro (KSP) repeats in NF-M and NF-H tail domains, se em to be restricted to axons, To explore the factors regulating the cellula r compartmentalization of NF phosphorylation, we separated cell bodies (GFL ) from axons in the squid stellate ganglion and compared the kinase activit y in the respective lysates, Although total kinase activity was similar in each lysate, the profile of endogenous phosphorylated substrates was striki ngly different. Neurofilament protein 220 (NF220), high-molecular-weight NF protein (HMW), and tubulin were the principal phosphorylated substrates in axoplasm, while tubulin was the principal GFL phosphorylated substrate, in addition to highly phosphorylated low-molecular-weight proteins. Western b lot analysis showed that whereas both lysates contained similar kinases and cytoskeletal proteins, phosphorylated NF220 and HMW were completely absent from the GFL lysate, These differences were highlighted by P13(suc1) affin ity chromatography, which revealed in axoplasm an active multimeric phospho rylation complex(es), enriched in cytoskeletal proteins and kinases; the eq uivalent P13 GFL complex exhibited six to 20 times less endogenous and exog enous phosphorylation activity, respectively, contained fewer cytoskeletal proteins and kinases, and expressed a qualitatively different cdc2-like kin ase epitope, 34 kDa rather than 49 kDa. Cell bodies and axons share a simil ar repertoire of molecular constitutents; however, the data suggest that th e cytoskeletal/kinase phosphorylation complexes extracted from each cellula r compartment by P13 are fundamentally different. (C) 1999 John Wiley & Son s, Inc.