Sharpin, a novel postsynaptic density protein that directly interacts with the Shank family of proteins
The Shank family of proteins(also termed cortBP, ProSAP, or Synamon) are highly enriched in the postsynaptic density(PSD) of excitatory synapses in brain. Shank contains multiple domains for protein-protein interactions including ankyrin repeats, SH3 domain, PDZ domain, SAM domain and an extensive proline-rich region. Proteins that bind to Shank have been identified: the PDZ domain binds to the GKAP/SAPAP family of proteins, the proline-rich region interacts with Homer/Vesl and cortactin, and the SAM domain mediates homomultimerization of Shank. The functions of the N-terminal ankyrin repeats and the SH3 domain of Shank are unknown. We have identified a novel protein, termed Sharpin, that directly interacts with the N-terminal region of Shank containing the ankyrin repeats. Sharpin is widely expressed at mRNA and protein levels in many tissues and enriched in the PSD in brain. Sharpin forms a complex with Shank in heterologous cells and can be coimmunoprecipitated with Shank, GKAP and PSD-95 from brain extracts. Immunostaining reveals the presence of Sharpin at excitatory synapses and its partial colocalization with Shank. While the C-terminal half of Sharpin interacts with Shank, the N-terminal half of Sharpin mediates homomultimerization. These results define a new component of the postsynaptic PSD-95/GKAP/Shank complex at excitatory synapses.