Shank protein

spinqualitylabelsall models Structure of Shank Family Proteins, 3l4f Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

      Shank Family Proteins are scaffolding proteins found in the postsynaptic density (PSD) of excitatory synapses. The PSD, a structure within dendritic spines that is associated with the postsynaptic membrane, contains a complex assembly of proteins which organize neurotransmitter receptors, signaling pathways, and regulatory elements within a cytoskeletal matrix.^[1] It aids the appropriate communication of incoming signals to cytoplasmic targets and contributes to neuronal plasticity by readily changing its composition and structure in response to neural activity.^[2] Shank proteins are believed to function as master organizer of the PSD owing to their ability to recruit and form multimeric complexes with postsynaptic receptors, signaling molecules and cytoskeletal proteins like AMPA, Neuroligin and NMDA glutamate receptors.^[3] Within the PSD, there are over 300 individual shank molecules in a single postsynaptic site, representing 5% of the total protein molecules and total protein mass of the PSD.^[4] Shanks contain five domains for protein-protein interactions, including an ankyrin repeat domain, used to bind acting regulating proteins, an Src homology 3 (Sh3) domain, used to bind AMPA receptors, a PDZ domain, used to bind G protein coupled receptors, several proline-rich domains , and a C-terminal SAM domain, which is responsible for mediating Shank multimerization.^[1] Functionally, Shank is involved in the maturation and enlargement of dendritic spines and is able to induce spine formation in neurons.^[3]

     Chromosome 22q13 deletion syndrome (22q13DS) is a neurobehavioral syndrome marked by neonatal hyptonia, global developmental delay, and autism spectrum disorder features.^[3] The SHANK3 gene is located within this region of chromosome 22. Studies have revealed that point mutations in SHANK3 can produce the entirety of neurodevelopmental symptoms associated with 22q13DS, accounting for 1% of autism cases.^[5] At the molecular level, disruption of the full length Shank3 protein results in reductions in AMPA receptor mediated transmission and spine remodeling.^[4] Shank3 heterozygous mice, who are haploinsufficient for the Shank3 gene display less social sniffing and emitted fewer ultrasonic vocalizations during interactions with estrus female. Further, knockout mice of Shank have a decreased spine number, a diminished PSD size, decreased levels of GKAP and Homer, and reduced synaptic transmission. Interestingly, overexpression of SHANK3 may also result in an ASD, supporting the hypothesis that Autism is caused by improper Excitatory/Inhibitory neuronal ratios in the brain.^[4] Measurements of broad miRNA expression levels in Autism patients uncovered dysregulated miRNAs for genes like that of MeCP2, the cause of Rett Syndrome, NRXN-1, a gene implicated in ASDs, and Shank3, validating Shank3’s role in autism.^[6] Due to the marked reduction in AMPA mediated transmission in Shank3 mutants, compounds that enhance AMPA transmission (AMPAkinses) as potential therapeutic approaches to treating some ASDs.^[4]

     βPIX belongs to a group of guanine nucleotide exchange factors used by Rho GTPase family members, like Rac1 and Cdc42, which are known to regulate the actin cytoskeleton of synapses.^[7] PIX has an N-terminal Src homology 3 (SH3) domain which associates with PAK, a coiled-coil (CC) domain, which is critical for multimerization, and a C-terminal PDZ binding domain which interacts with the PDZ domain of Shank.^[7] The interaction of Shank with βPIX promotes the synaptic localization of βPIX and βPIX associated p21 Associated Kinase (PAK). Since PAK is known to regulate the actin cytoskeleton and that dendritic spines are actin-rich structures, it is believed that Shank recruits βPIX and associated proteins to spines and regulates postsynaptic structure.^[1]

     The **canonical PDZ domain** contains 90 amino acids and folds into a compact **globular structure** consisting of a six-stranded β-sandwich flanked by two alpha helices.^[7] βPIX forms a **parallel trimer** via **helical interactions** within its CC domain, and with a **PDZ binding domain** at the C-terminus. Interestingly, only 1 Shank molecule is bound to the CC domain trimer of βPIX in an **asymettric assembly**. (SHOW ZOOMED OUT IN SPACE FILL WITH LONG PART DIRECTLY VERTICAL) The **8-residue PDZ binding domain** (BALL AND STICK AND SPHERE COMBO BURIED MODE) of βPIX forms a number of **hydrogen bonding and hydrophobic interactions** (FIGURE 2A) with the Shank PDZ domain. Shank3-Arg 679 forms the **most critical interaction** with βPIX, tightly binding Glutamate -3. Abolishing this interaction through mutagenesis completely eliminates the assembly. Upon binding of βPIX, the PDZ domain undergoes a significant **conformational change** (OVERVIEW MORPH). Lys 682 undergoes a nearly **11 Angstrom displacement**, ultimately forming a **beta-sheet interaction**, with βPIX residues -4--6, incorporating Shank residues 680 and 681.^[7]

     Shank proteins are wedged between scaffolding proteins that are bound to either neurotransmitter receptors or the actin cytoskeleton, making them well positioned to nucleate the underlying structure of the PSD.^[2] The SAM domain of () to form large sheets composed of helical fibers stacked side by side. The proposed sheet structure with radially projecting protein interaction domains, appears to be an ideal architecture for a protein that must contact both membrane and cytoplasmic components at a cell surface.^[2] Models of this sort validate the importance of Shank3 as master scaffolding proteins and illustrate how slight mutations can disrupt an entire PSD and synaptic function.