Shank protein

     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]      BetaPIX 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 betaPIX promotes the synaptic localization of betaPIX and betaPIX 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 betaPIX 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 beta-sandwich flanked by two alpha helices.^[7] BetaPIX forms a **parallel trimer** via its CC domain with a **PDZ binding domain** at the C-terminus. Interestingly, only 1 Shank molecule is bound to the CC domain trimer of betaPIX 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 BetaPIX 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 BetaPIX, tightly binding Glutamate -3. Abolishing this interaction through mutagenesis completely eliminates the assembly. Upon binding of betaPIX, 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 betaPIX 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 **Shank3 can oligomerize** 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. A number of hydrogen bonding and hydrophobic interactions stabilize the **intra-polymer** interface and the inter-polymer interfaces, labeled **contact 1** and **contact 2**.^[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.