Association of synapsin I with neuronal cytoskeleton. Identification in cytoskeletal preparations in vitro and immunocytochemical localization in brain of synapsin I.

Calmodulin-dependent protein kinase II (CaM kinase II) is associated with microtubule preparations and phosphorylates several endogenous proteins including microtubule-associated protein 2, tubulin, and an 80,000-dalton protein doublet (pp80). We now report that pp80 is identical to synapsin I by all criteria studied including molecular weight, isoelectric point, phosphopeptide mapping of cAMP- and calmodulin-dependent phosphorylated protein, comigration with authentic synapsin I, and sensitivity to digestion with collagenase. Synapsin I and CaM kinase II were found in association with both microtubule preparations and preparations enriched in neurofilaments. Antibodies to synapsin I specifically labeled neurofilaments prepared in vitro. Immunocytochemical studies on rat brain tissue demonstrated synapsin I immunoreactivity specifically associated with the neuronal cytoskeleton as well as synaptic vesicles. The observed synapsin I staining on cytoskeletal elements was considerably diminished or abolished by the inclusion of Triton X-100 in the staining solutions. These results indicate that synapsin I is associated with the cytoskeleton and may be an important link between cytoskeletal elements as well as between the cytoskeleton and membrane.