Abstract
Endogenous phosphorylation of synapsin I (protein I), a phosphoprotein located on the surface of synaptic vesicles, was studied in vesicles prepared from synaptosomes lysed in the absence (control) or presence of 50 μM-cyclic AMP (“cAMP-treated”). Compared to synaptic plasma membrane (SPM) fractions prepared in parallel, and confirming previous work, the vesicle fractions were highly enriched on a unit protein basis in Ca2+-calmodulin-dependent kinase activity towards synapsin I. In contrast, with control vesicles the magnitude of the total phosphorylation of synapsin I in the presence of cyclic AMP was similar to that observed in SPM, but regulation by cyclic AMP was only partial. In “cAMP-treated” vesicles, however, synapsin I phosphorylation was highly enriched compared to SPM and the activity was virtually independent of cyclic AMP. The results show that while the free catalytic subunit of the cyclic AMP-dependent kinase remains associated with synapsin I during vesicle isolation the holoenzyme remains bound to membrane fragments, probably through its regulatory subunit.
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Dedicated to Henry McIlwain.
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Rodnight, R., Gower, H.J. & Robertson, R.G. On the disposition of a phosphorylated protein (“synapsin I”) and its associated kinases in synaptosomes from rat brain. Neurochem Res 9, 771–783 (1984). https://doi.org/10.1007/BF00965665
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DOI: https://doi.org/10.1007/BF00965665