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Phosphorylation-dependent Effects of Synapsin IIa on Actin Polymerization and Network Formation

Authors

  • Hendrikus B. Nielander,

    1. Department of Experimental Medicine, University of Roma Tor Vergata, 00133 Roma and Department of Biomedical Sciences, Section of Physiology, University of Modena, 41100 Modena, Italy
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  • Franco Onofri,

    1. Department of Experimental Medicine, University of Roma Tor Vergata, 00133 Roma and Department of Biomedical Sciences, Section of Physiology, University of Modena, 41100 Modena, Italy
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  • Eric Schaeffer,

    1. Laboratory of Molecular and Cellular Neuroscience, The Rockefeller University, New York, NY 10021–6399, USA
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  • Andrea Menegon,

    1. DIBIT, S.Raffaele Sci. Institute, ‘B.Ceccarelli’ and CNR Cellular and Molecular Pharmacology Centers, Department of Medical Pharmacology, University of Milano, 20100 Milano, Italy
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  • Riccardo Fesce,

    1. DIBIT, S.Raffaele Sci. Institute, ‘B.Ceccarelli’ and CNR Cellular and Molecular Pharmacology Centers, Department of Medical Pharmacology, University of Milano, 20100 Milano, Italy
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  • Flavia Valtorta,

    1. DIBIT, S.Raffaele Sci. Institute, ‘B.Ceccarelli’ and CNR Cellular and Molecular Pharmacology Centers, Department of Medical Pharmacology, University of Milano, 20100 Milano, Italy
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  • Paul Greengard,

    1. Laboratory of Molecular and Cellular Neuroscience, The Rockefeller University, New York, NY 10021–6399, USA
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  • Fabio Benfenati

    Corresponding author
    1. Department of Experimental Medicine, University of Roma Tor Vergata, 00133 Roma and Department of Biomedical Sciences, Section of Physiology, University of Modena, 41100 Modena, Italy
    • F. Benfenati, Department of Biomedical Sciences, Section of Physiology, Via Campi 287, 1–41100 Modena, Italy

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Abstract

The synapsins are a family of synaptic vesicle phosphoproteins which play a key role in the regulation of neurotransmitter release and synapse formation. In the case of synapsin I, these biological properties have been attributed to its ability to interact with both synaptic vesicles and the actin-based cytoskeleton. Although synapsin II shares some of the biological properties of synapsin I, much less is known of its molecular properties. We have investigated the interactions of recombinant rat synapsin Ila with monomeric and filamentous actin and the sensitivity of those interactions to phosphorylation, and found that: i) dephosphotylated synapsin II stimulates actin polymerization by binding to actin monomers and forming actively elongating nuclei and by facilitating the spontaneous nucleation/elongation processes; ii) dephosphorylated synapsin II induces the formation of thick and ordered bundles of actin filaments with greater potency than synapsin I; iii) phosphorylation by protein kinase A markedly inhibits the ability of synapsin II to interact with both actin monomers and filaments. The results indicate that the interactions of synapsin II with actin are similar but not identical to those of synapsin I and suggest that synapsin II may play a major structural role in mature and developing nerve terminals, which is only partially overlapping with the role played by synapsin I.

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