Phosphorylation by CaM kinase II modulates phospholipid-binding activity of p65 protein in synaptic vesicles

Synaptotagmins are Ca2+-and phospholipid-binding proteins of synaptic vesicles that might function as Ca2+ receptors for neurotransmitter release via their first C2 (C2A) domain. Here we describe the effect of Mg2+ on phospholipid binding to the C2A domains of multiple synaptotagmins (II–VI), and demonstrate that only synaptotagmin III can bind negatively charged phospholipids [phosphatidylserine (PS) and phosphatidylinositol] in a Mg2+-dependent manner. The Mg2+-dependent interaction with PS was found to have an EC50 of approx. 30 μM Mg2+, which is comparable to that of Sr2+ and Ba2+ (EC50 values of approx. 10 μM). This binding property of the C2A domain is specific to synaptotagmin III, because none of the C2A domains of other proteins, such as rabphilin 3A, Doc2α, Doc2β or Gap1m, showed phospholipid binding activity in the presence of 1 mM Mg2+. Our results suggest that synaptotagmin III is involved in presynaptic functions different from those of synaptotagmins I and II.

Download Full-text

Dephosphorylated synapsin I anchors synaptic vesicles to actin cytoskeleton: an analysis by videomicroscopy.

The Journal of Cell Biology ◽

10.1083/jcb.128.5.905 ◽

1995 ◽

Vol 128 (5) ◽

pp. 905-912 ◽

Cited By ~ 93

Author(s):

P E Ceccaldi ◽

F Grohovaz ◽

F Benfenati ◽

E Chieregatti ◽

P Greengard ◽

...

Keyword(s):

Actin Cytoskeleton ◽

Synaptic Vesicle ◽

Nerve Terminal ◽

Actin Filaments ◽

Neurotransmitter Release ◽

Synaptic Vesicles ◽

Actin Polymerization ◽

Synapsin I ◽

Cam Kinase Ii ◽

Cam Kinase

Synapsin I is a synaptic vesicle-associated protein which inhibits neurotransmitter release, an effect which is abolished upon its phosphorylation by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Based on indirect evidence, it was suggested that this effect on neurotransmitter release may be achieved by the reversible anchoring of synaptic vesicles to the actin cytoskeleton of the nerve terminal. Using video-enhanced microscopy, we have now obtained experimental evidence in support of this model: the presence of dephosphorylated synapsin I is necessary for synaptic vesicles to bind actin; synapsin I is able to promote actin polymerization and bundling of actin filaments in the presence of synaptic vesicles; the ability to cross-link synaptic vesicles and actin is specific for synapsin I and is not shared by other basic proteins; the cross-linking between synaptic vesicles and actin is specific for the membrane of synaptic vesicles and does not reflect either a non-specific binding of membranes to the highly surface active synapsin I molecule or trapping of vesicles within the thick bundles of actin filaments; the formation of the ternary complex is virtually abolished when synapsin I is phosphorylated by CaM kinase II. The data indicate that synapsin I markedly affects synaptic vesicle traffic and cytoskeleton assembly in the nerve terminal and provide a molecular basis for the ability of synapsin I to regulate the availability of synaptic vesicles for exocytosis and thereby the efficiency of neurotransmitter release.

Download Full-text