Linkage of the Actin Cytoskeleton to the Postsynaptic Density via Direct Interactions of Abp1 with the ProSAP/Shank Family

Synaptic contacts contain elaborate cytomatrices on both sides of the synaptic cleft, which are believed to organize and link the different synaptic functions in time and space and can respond to different inner and outer cues with massive structural reorganizations. At the PSD (postsynaptic density), activity-dependent reorganizations of the cortical actin cytoskeleton are hypothesized to play a role in synaptic plasticity. Here, we report on interactions of the F-actin binding protein Abp1 with members of the ProSAP/Shank family: multidomain scaffolding PSD proteins interconnecting glutamate receptors with other synaptic components. Affinity-purification experiments demonstrate that the interactions are mediated by the Abp1 (actin-binding protein 1) SH3 (Src homology 3) domain, which associates with a proline-rich motif that is conserved within the C-terminal parts of ProSAP1(proline-rich synapse-associated protein 1)/Shank2 and ProSAP2/Shank3. The distribution of Abp1, ProSAP1, and ProSAP2 overlaps within the brain, and all three proteins are part of the PSD and are particularly enriched in cortex and hippocampus. Coimmunoprecipitation of endogenous Abp1 and ProSAP2 and colocalization studies of Abp1 and ProSAPs in hippocampal neurons indicate the in vivo relevance of the interactions. Intriguingly, in vivo recruitment assays demonstrate that Abp1 can bind to dynamic F-actin structures and ProSAPs simultaneously, suggesting that Abp1 might link different organizing elements in the PSD. Importantly, different paradigms of neuronal stimulation induce a redistribution of Abp1 to ProSAP-containing synapses. Our data suggest that ProSAPs may serve to localize Abp1 to dendritic spines, thus serving as attachment points for the dynamic postsynaptic cortical actin cytoskeleton. This creates a functional connection between synaptic stimulation and cytoskeletal rearrangements.