cAMP-dependent phosphorylation of neurofilament proteins NF-L and NF-M inhibits their coassembly into filaments in vitro.

The effects of cAMP-dependent phosphorylation of neurofilament proteins NF-L and NF-M on the coassembly of the two proteins into filaments in vitro was examined. Sedimentation velocity experiments revealed that phosphorylated NF-M sedimented more slowly and resisted salt-induced aggregation. Filaments reconstituted from various mixtures of proteins were pelleted were pelleted and examined by gel electrophoresis and electron microscopy. Phosphorylation of either protein inhibited the formation of heteropolymer filaments containing NF-L and NF-M. However, phosphorylated proteins were fully competent in forming heterooligomeric assembly intermediates; therefore, phosphorylation blocks a later stage of filament assembly.